diff --git a/dom/system/gonk/ril_worker.js b/dom/system/gonk/ril_worker.js
index 29bb1a7be2fc..79479efbc248 100644
--- a/dom/system/gonk/ril_worker.js
+++ b/dom/system/gonk/ril_worker.js
@@ -244,16 +244,6 @@ RilObject.prototype = {
      */
     this._pendingNetworkInfo = {rilMessageType: "networkinfochanged"};
 
-    /**
-     * USSD session flag.
-     * Only one USSD session may exist at a time, and the session is assumed
-     * to exist until:
-     *    a) There's a call to cancelUSSD()
-     *    b) The implementation sends a UNSOLICITED_ON_USSD with a type code
-     *       of "0" (USSD-Notify/no further action) or "2" (session terminated)
-     */
-    this._ussdSession = null;
-
     /**
      * Cell Broadcast Search Lists.
      */
@@ -1845,67 +1835,13 @@ RilObject.prototype = {
     this.context.Buf.simpleRequest(REQUEST_LAST_CALL_FAIL_CAUSE, options);
   },
 
-  sendMMI: function(options) {
-    if (DEBUG) {
-      this.context.debug("SendMMI " + JSON.stringify(options));
-    }
-
-    let _sendMMIError = (function(errorMsg) {
-      options.errorMsg = errorMsg;
-      this.sendChromeMessage(options);
-    }).bind(this);
-
-    // It's neither a valid mmi code nor an ongoing ussd.
-    let mmi = options.mmi;
-    if (!mmi && !this._ussdSession) {
-      _sendMMIError(MMI_ERROR_KS_ERROR);
-      return;
-    }
-
-    let _isRadioAvailable = (function() {
-      if (this.radioState !== GECKO_RADIOSTATE_ENABLED) {
-        _sendMMIError(GECKO_ERROR_RADIO_NOT_AVAILABLE);
-        return false;
-      }
-      return true;
-    }).bind(this);
-
-    // If the MMI code is not a known code, it is treated as an ussd.
-    if (!_isRadioAvailable()) {
-      return;
-    }
-
-    options.ussd = mmi.fullMMI;
-
-    if (this._ussdSession) {
-      if (DEBUG) this.context.debug("Cancel existing ussd session.");
-      this.cachedUSSDRequest = options;
-      this.cancelUSSD({});
-      return;
-    }
-
-    this.sendUSSD(options, false);
-  },
-
-  /**
-   * Cache the request for send out a new ussd when there is an existing
-   * session. We should do cancelUSSD first.
-   */
-  cachedUSSDRequest : null,
-
   /**
    * Send USSD.
    *
    * @param ussd
    *        String containing the USSD code.
    */
-  sendUSSD: function(options, checkSession = true) {
-    if (checkSession && !this._ussdSession) {
-      options.errorMsg = GECKO_ERROR_GENERIC_FAILURE;
-      this.sendChromeMessage(options);
-      return;
-    }
-
+  sendUSSD: function(options) {
     let Buf = this.context.Buf;
     Buf.newParcel(REQUEST_SEND_USSD, options);
     Buf.writeString(options.ussd);
@@ -4360,24 +4296,12 @@ RilObject.prototype[REQUEST_SEND_USSD] = function REQUEST_SEND_USSD(length, opti
   if (DEBUG) {
     this.context.debug("REQUEST_SEND_USSD " + JSON.stringify(options));
   }
-  this._ussdSession = !options.errorMsg;
   this.sendChromeMessage(options);
 };
 RilObject.prototype[REQUEST_CANCEL_USSD] = function REQUEST_CANCEL_USSD(length, options) {
   if (DEBUG) {
     this.context.debug("REQUEST_CANCEL_USSD" + JSON.stringify(options));
   }
-
-  this._ussdSession = !!options.errorMsg;
-
-  // The cancelUSSD is triggered by ril_worker itself.
-  if (this.cachedUSSDRequest) {
-    if (DEBUG) this.context.debug("Send out the cached ussd request");
-    this.sendUSSD(this.cachedUSSDRequest);
-    this.cachedUSSDRequest = null;
-    return;
-  }
-
   this.sendChromeMessage(options);
 };
 RilObject.prototype[REQUEST_GET_CLIR] = function REQUEST_GET_CLIR(length, options) {
@@ -5166,18 +5090,12 @@ RilObject.prototype[UNSOLICITED_ON_USSD] = function UNSOLICITED_ON_USSD() {
     this.context.debug("On USSD. Type Code: " + typeCode + " Message: " + message);
   }
 
-  let oldSession = this._ussdSession;
-
-  // Per ril.h the USSD session is assumed to persist if the type code is "1".
-  this._ussdSession = typeCode == "1";
-
-  if (!oldSession && !this._ussdSession && !message) {
-    return;
-  }
-
   this.sendChromeMessage({rilMessageType: "ussdreceived",
                           message: message,
-                          sessionEnded: !this._ussdSession});
+                          // Per ril.h the USSD session is assumed to persist if
+                          // the type code is "1", otherwise the current session
+                          // (if any) is assumed to have terminated.
+                          sessionEnded: typeCode !== "1"});
 };
 RilObject.prototype[UNSOLICITED_ON_USSD_REQUEST] = null;
 RilObject.prototype[UNSOLICITED_NITZ_TIME_RECEIVED] = function UNSOLICITED_NITZ_TIME_RECEIVED() {
diff --git a/dom/system/gonk/tests/test_ril_worker_mmi.js b/dom/system/gonk/tests/test_ril_worker_mmi.js
deleted file mode 100644
index de52c2e61af0..000000000000
--- a/dom/system/gonk/tests/test_ril_worker_mmi.js
+++ /dev/null
@@ -1,118 +0,0 @@
-/* Any copyright is dedicated to the Public Domain.
- * http://creativecommons.org/publicdomain/zero/1.0/ */
-
-subscriptLoader.loadSubScript("resource://gre/modules/ril_consts.js", this);
-
-function run_test() {
-  run_next_test();
-}
-
-function createMMIOptions(procedure, serviceCode, sia, sib, sic) {
-  let mmi = {
-    fullMMI: Array.slice(arguments).join("*") + "#",
-    procedure: procedure,
-    serviceCode: serviceCode,
-    sia: sia,
-    sib: sib,
-    sic: sic
-  };
-
-  return mmi;
-}
-
-function testSendMMI(mmi, error) {
-  let workerhelper = newInterceptWorker();
-  let worker = workerhelper.worker;
-  let context = worker.ContextPool._contexts[0];
-
-  do_print("worker.postMessage " + worker.postMessage);
-
-  context.RIL.radioState = GECKO_RADIOSTATE_ENABLED;
-  context.RIL.sendMMI({rilMessageType: "sendMMI", mmi: mmi});
-
-  let postedMessage = workerhelper.postedMessage;
-
-  equal(postedMessage.rilMessageType, "sendMMI");
-  equal(postedMessage.errorMsg, error);
-}
-
-/**
- * sendMMI tests.
- */
-
-add_test(function test_sendMMI_null() {
-  testSendMMI(null, MMI_ERROR_KS_ERROR);
-
-  run_next_test();
-});
-
-add_test(function test_sendMMI_short_code() {
-  let workerhelper = newInterceptWorker();
-  let worker = workerhelper.worker;
-  let context = worker.ContextPool._contexts[0];
-
-  let ussdOptions;
-
-  context.RIL.sendUSSD = function fakeSendUSSD(options){
-    ussdOptions = options;
-    context.RIL[REQUEST_SEND_USSD](0, {});
-  };
-
-  context.RIL.radioState = GECKO_RADIOSTATE_ENABLED;
-  context.RIL.sendMMI({mmi: {fullMMI: "**"}});
-
-  let postedMessage = workerhelper.postedMessage;
-  equal(ussdOptions.ussd, "**");
-  equal(postedMessage.errorMsg, undefined);
-  ok(context.RIL._ussdSession);
-
-  run_next_test();
-});
-
-add_test(function test_sendMMI_USSD() {
-  let workerhelper = newInterceptWorker();
-  let worker = workerhelper.worker;
-  let context = worker.ContextPool._contexts[0];
-  let ussdOptions;
-
-  context.RIL.sendUSSD = function fakeSendUSSD(options) {
-    ussdOptions = options;
-    context.RIL[REQUEST_SEND_USSD](0, {});
-  };
-
-  context.RIL.radioState = GECKO_RADIOSTATE_ENABLED;
-  context.RIL.sendMMI({mmi: createMMIOptions("*", "123")});
-
-  let postedMessage = workerhelper.postedMessage;
-
-  equal(ussdOptions.ussd, "**123#");
-  equal(postedMessage.errorMsg, undefined);
-  ok(context.RIL._ussdSession);
-
-  run_next_test();
-});
-
-add_test(function test_sendMMI_USSD_error() {
-  let workerhelper = newInterceptWorker();
-  let worker = workerhelper.worker;
-  let context = worker.ContextPool._contexts[0];
-  let ussdOptions;
-
-  context.RIL.sendUSSD = function fakeSendUSSD(options){
-    ussdOptions = options;
-    context.RIL[REQUEST_SEND_USSD](0, {
-      errorMsg: GECKO_ERROR_GENERIC_FAILURE
-    });
-  };
-
-  context.RIL.radioState = GECKO_RADIOSTATE_ENABLED;
-  context.RIL.sendMMI({mmi: createMMIOptions("*", "123")});
-
-  let postedMessage = workerhelper.postedMessage;
-
-  equal(ussdOptions.ussd, "**123#");
-  equal(postedMessage.errorMsg, GECKO_ERROR_GENERIC_FAILURE);
-  ok(!context.RIL._ussdSession);
-
-  run_next_test();
-});
diff --git a/dom/system/gonk/tests/xpcshell.ini b/dom/system/gonk/tests/xpcshell.ini
index 661dd16d1963..6af3d7437f06 100644
--- a/dom/system/gonk/tests/xpcshell.ini
+++ b/dom/system/gonk/tests/xpcshell.ini
@@ -23,7 +23,6 @@ skip-if = true
 [test_ril_worker_sms_gsmpduhelper.js]
 [test_ril_worker_sms_segment_info.js]
 [test_ril_worker_smsc_address.js]
-[test_ril_worker_mmi.js]
 [test_ril_worker_cf.js]
 [test_ril_worker_cellbroadcast_config.js]
 [test_ril_worker_cellbroadcast.js]
diff --git a/dom/telephony/gonk/TelephonyService.js b/dom/telephony/gonk/TelephonyService.js
index eab339d67d5c..aa09586510ce 100644
--- a/dom/telephony/gonk/TelephonyService.js
+++ b/dom/telephony/gonk/TelephonyService.js
@@ -230,6 +230,7 @@ function TelephonyService() {
   this._currentCalls = {};
   this._currentConferenceState = nsITelephonyService.CALL_STATE_UNKNOWN;
   this._audioStates = [];
+  this._ussdSessions = [];
 
   this._cdmaCallWaitingNumber = null;
 
@@ -243,6 +244,7 @@ function TelephonyService() {
 
   for (let i = 0; i < this._numClients; ++i) {
     this._audioStates[i] = nsITelephonyAudioService.PHONE_STATE_NORMAL;
+    this._ussdSessions[i] = false;
     this._currentCalls[i] = {};
     this._enumerateCallsForClient(i);
   }
@@ -265,6 +267,15 @@ TelephonyService.prototype = {
   _callRingWakeLock: null,
   _callRingWakeLockTimer: null,
 
+  /**
+   * USSD session flags.
+   * Only one USSD session may exist at a time, and the session is assumed
+   * to exist until:
+   *    a) There's a call to cancelUSSD()
+   *    b) Receiving a session end unsolicited event.
+   */
+  _ussdSessions: null,
+
   _acquireCallRingWakeLock: function() {
     if (!this._callRingWakeLock) {
       if (DEBUG) debug("Acquiring a CPU wake lock for handling incoming call.");
@@ -916,53 +927,25 @@ TelephonyService.prototype = {
         this._callWaitingMMI(aClientId, aMmi, aCallback);
         break;
 
-      // Fall back to "sendMMI".
+      // Handle unknown MMI code as USSD.
       default:
-        this._sendMMI(aClientId, aMmi, aCallback);
+        if (!this._isRadioOn(aClientId)) {
+          aCallback.notifyDialMMIError(RIL.GECKO_ERROR_RADIO_NOT_AVAILABLE);
+          return;
+        }
+
+        this._sendUSSDInternal(aClientId, aMmi.fullMMI, aResponse => {
+          if (aResponse.errorMsg) {
+            aCallback.notifyDialMMIError(aResponse.errorMsg);
+            return;
+          }
+
+          aCallback.notifyDialMMISuccess("");
+        });
         break;
     }
   },
 
-  _sendMMI: function(aClientId, aMmi, aCallback) {
-    this._sendToRilWorker(aClientId, "sendMMI",
-                          { mmi: aMmi }, response => {
-      if (DEBUG) debug("MMI response: " + JSON.stringify(response));
-
-      if (response.errorMsg) {
-        if (response.additionalInformation != null) {
-          aCallback.notifyDialMMIErrorWithInfo(response.errorMsg,
-                                               response.additionalInformation);
-        } else {
-          aCallback.notifyDialMMIError(response.errorMsg);
-        }
-        return;
-      }
-
-      // No additional information
-      if (response.additionalInformation === undefined) {
-        aCallback.notifyDialMMISuccess(response.statusMessage);
-        return;
-      }
-
-      // Additional information is an integer.
-      if (!isNaN(parseInt(response.additionalInformation, 10))) {
-        aCallback.notifyDialMMISuccessWithInteger(
-          response.statusMessage, response.additionalInformation);
-        return;
-      }
-
-      // Additional information is an array of strings.
-      let array = response.additionalInformation;
-      if (Array.isArray(array) && array.length > 0 && typeof array[0] === "string") {
-        aCallback.notifyDialMMISuccessWithStrings(response.statusMessage,
-                                                  array.length, array);
-        return;
-      }
-
-      aCallback.notifyDialMMISuccess(response.statusMessage);
-    });
-  },
-
   /**
    * Handle call forwarding MMI code.
    *
@@ -2015,13 +1998,42 @@ TelephonyService.prototype = {
   },
 
   sendUSSD: function(aClientId, aUssd, aCallback) {
-    this._sendToRilWorker(aClientId, "sendUSSD", { ussd: aUssd },
-                          this._defaultCallbackHandler.bind(this, aCallback));
+    this._sendUSSDInternal(aClientId, aUssd,
+                           this._defaultCallbackHandler.bind(this, aCallback));
+  },
+
+  _sendUSSDInternal: function(aClientId, aUssd, aCallback) {
+    if (!this._ussdSessions[aClientId]) {
+      this._sendToRilWorker(aClientId, "sendUSSD", { ussd: aUssd }, aResponse => {
+        this._ussdSessions[aClientId] = !aResponse.errorMsg;
+        aCallback(aResponse);
+      });
+      return;
+    }
+
+    // Cancel the previous ussd session first.
+    this._cancelUSSDInternal(aClientId, aResponse => {
+      // Fail to cancel ussd session, report error instead of sending ussd
+      // request.
+      if (aResponse.errorMsg) {
+        aCallback(aResponse);
+        return;
+      }
+
+      this._sendUSSDInternal(aClientId, aUssd, aCallback);
+    });
   },
 
   cancelUSSD: function(aClientId, aCallback) {
-    this._sendToRilWorker(aClientId, "cancelUSSD", {},
-                          this._defaultCallbackHandler.bind(this, aCallback));
+    this._cancelUSSDInternal(aClientId,
+                             this._defaultCallbackHandler.bind(this, aCallback));
+  },
+
+  _cancelUSSDInternal: function(aClientId, aCallback) {
+    this._sendToRilWorker(aClientId, "cancelUSSD", {}, aResponse => {
+      this._ussdSessions[aClientId] = !!aResponse.errorMsg;
+      aCallback(aResponse);
+    });
   },
 
   get microphoneMuted() {
@@ -2289,6 +2301,13 @@ TelephonyService.prototype = {
             aMessage + " (sessionEnded : " + aSessionEnded + ")");
     }
 
+    let oldSession = this._ussdSessions[aClientId];
+    this._ussdSessions[aClientId] = !aSessionEnded;
+
+    if (!oldSession && !this._ussdSessions[aClientId] && !aMessage) {
+      return;
+    }
+
     gTelephonyMessenger.notifyUssdReceived(aClientId, aMessage, aSessionEnded);
   },