diff --git a/src/CallHandler.tsx b/src/CallHandler.tsx
index 8fd45c49ae2..fe938c99295 100644
--- a/src/CallHandler.tsx
+++ b/src/CallHandler.tsx
@@ -485,8 +485,8 @@ export default class CallHandler extends EventEmitter {
                 this.pause(AudioID.Ringback);
             }
 
-            this.calls.set(mappedRoomId, newCall);
-            this.emit(CallHandlerEvent.CallsChanged, this.calls);
+            this.removeCallForRoom(mappedRoomId);
+            this.addCallForRoom(mappedRoomId, newCall);
             this.setCallListeners(newCall);
             this.setCallState(newCall, newCall.state);
         });
@@ -521,8 +521,7 @@ export default class CallHandler extends EventEmitter {
                     this.removeCallForRoom(mappedRoomId);
                     mappedRoomId = newMappedRoomId;
                     console.log("Moving call to room " + mappedRoomId);
-                    this.calls.set(mappedRoomId, call);
-                    this.emit(CallHandlerEvent.CallChangeRoom, call);
+                    this.addCallForRoom(mappedRoomId, call, true);
                 }
             }
         });
@@ -756,9 +755,15 @@ export default class CallHandler extends EventEmitter {
         console.log("Current turn creds expire in " + timeUntilTurnCresExpire + " ms");
         const call = MatrixClientPeg.get().createCall(mappedRoomId);
 
-        console.log("Adding call for room ", roomId);
-        this.calls.set(roomId, call);
-        this.emit(CallHandlerEvent.CallsChanged, this.calls);
+        try {
+            this.addCallForRoom(roomId, call);
+        } catch (e) {
+            Modal.createTrackedDialog('Call Handler', 'Existing Call with user', ErrorDialog, {
+                title: _t('Already in call'),
+                description: _t("You're already in a call with this person."),
+            });
+            return;
+        }
         if (transferee) {
             this.transferees[call.callId] = transferee;
         }
@@ -810,13 +815,8 @@ export default class CallHandler extends EventEmitter {
                         return;
                     }
 
-                    if (this.getCallForRoom(room.roomId)) {
-                        Modal.createTrackedDialog('Call Handler', 'Existing Call with user', ErrorDialog, {
-                            title: _t('Already in call'),
-                            description: _t("You're already in a call with this person."),
-                        });
-                        return;
-                    }
+                    // We leave the check for whether there's already a call in this room until later,
+                    // otherwise it can race.
 
                     const members = room.getJoinedMembers();
                     if (members.length <= 1) {
@@ -870,9 +870,8 @@ export default class CallHandler extends EventEmitter {
                     }
 
                     Analytics.trackEvent('voip', 'receiveCall', 'type', call.type);
-                    console.log("Adding call for room ", mappedRoomId);
-                    this.calls.set(mappedRoomId, call);
-                    this.emit(CallHandlerEvent.CallsChanged, this.calls);
+
+                    this.addCallForRoom(mappedRoomId, call);
                     this.setCallListeners(call);
                     // Explicitly handle first state change
                     this.onCallStateChanged(call.state, null, call);
@@ -1150,4 +1149,21 @@ export default class CallHandler extends EventEmitter {
             messaging.transport.send(ElementWidgetActions.HangupCall, {});
         });
     }
+
+    private addCallForRoom(roomId: string, call: MatrixCall, changedRooms = false): void {
+        if (this.calls.has(roomId)) {
+            console.log(`Couldn't add call to room ${roomId}: already have a call for this room`);
+            throw new Error("Already have a call for room " + roomId);
+        }
+
+        console.log("setting call for room " + roomId);
+        this.calls.set(roomId, call);
+
+        // Should we always emit CallsChanged too?
+        if (changedRooms) {
+            this.emit(CallHandlerEvent.CallChangeRoom, call);
+        } else {
+            this.emit(CallHandlerEvent.CallsChanged, this.calls);
+        }
+    }
 }
diff --git a/src/i18n/strings/en_EN.json b/src/i18n/strings/en_EN.json
index 2cb0a546aaf..0af0b971d2e 100644
--- a/src/i18n/strings/en_EN.json
+++ b/src/i18n/strings/en_EN.json
@@ -52,12 +52,12 @@
     "A microphone and webcam are plugged in and set up correctly": "A microphone and webcam are plugged in and set up correctly",
     "Permission is granted to use the webcam": "Permission is granted to use the webcam",
     "No other application is using the webcam": "No other application is using the webcam",
+    "Already in call": "Already in call",
+    "You're already in a call with this person.": "You're already in a call with this person.",
     "VoIP is unsupported": "VoIP is unsupported",
     "You cannot place VoIP calls in this browser.": "You cannot place VoIP calls in this browser.",
     "Too Many Calls": "Too Many Calls",
     "You've reached the maximum number of simultaneous calls.": "You've reached the maximum number of simultaneous calls.",
-    "Already in call": "Already in call",
-    "You're already in a call with this person.": "You're already in a call with this person.",
     "You cannot place a call with yourself.": "You cannot place a call with yourself.",
     "Unable to look up phone number": "Unable to look up phone number",
     "There was an error looking up the phone number": "There was an error looking up the phone number",