diff --git a/src/matrix/calls/CallHandler.ts b/src/matrix/calls/CallHandler.ts
index 7cd60208..f60c1fe6 100644
--- a/src/matrix/calls/CallHandler.ts
+++ b/src/matrix/calls/CallHandler.ts
@@ -50,11 +50,13 @@ export class CallHandler {
     // map of userId to set of conf_id's they are in
     private memberToCallIds: Map<string, Set<string>> = new Map();
     private groupCallOptions: GroupCallOptions;
+    private sessionId = makeId("s");
 
     constructor(private readonly options: Options) {
         this.groupCallOptions = Object.assign({}, this.options, {
             emitUpdate: (groupCall, params) => this._calls.update(groupCall.id, params),
             createTimeout: this.options.clock.createTimeout,
+            sessionId: this.sessionId
         });
     }
 
diff --git a/src/matrix/calls/PeerCall.ts b/src/matrix/calls/PeerCall.ts
index b85cee61..1b869f47 100644
--- a/src/matrix/calls/PeerCall.ts
+++ b/src/matrix/calls/PeerCall.ts
@@ -61,6 +61,7 @@ export class PeerCall implements IDisposable {
     private _state = CallState.Fledgling;
     private direction: CallDirection;
     private localMedia?: LocalMedia;
+    private seq: number = 0;
     // A queue for candidates waiting to go out.
     // We try to amalgamate candidates into a single candidate message where
     // possible
@@ -258,6 +259,7 @@ export class PeerCall implements IDisposable {
         const content = {
             call_id: callId,
             version: 1,
+            seq: this.seq++,
         };
         // TODO: Don't send UserHangup reason to older clients
         if (reason) {
@@ -304,6 +306,7 @@ export class PeerCall implements IDisposable {
                 offer,
                 [SDPStreamMetadataKey]: this.localMedia!.getSDPMetadata(),
                 version: 1,
+                seq: this.seq++,
                 lifetime: CALL_TIMEOUT_MS
             };
             if (this._state === CallState.CreateOffer) {
@@ -578,6 +581,7 @@ export class PeerCall implements IDisposable {
         const answerContent: MCallAnswer<MCallBase> = {
             call_id: this.callId,
             version: 1,
+            seq: this.seq++,
             answer: {
                 sdp: localDescription.sdp,
                 type: localDescription.type,
@@ -648,6 +652,7 @@ export class PeerCall implements IDisposable {
                     content: {
                         call_id: this.callId,
                         version: 1,
+                        seq: this.seq++,
                         candidates
                     },
                 }, log);
diff --git a/src/matrix/calls/callEventTypes.ts b/src/matrix/calls/callEventTypes.ts
index a32a7739..9189911f 100644
--- a/src/matrix/calls/callEventTypes.ts
+++ b/src/matrix/calls/callEventTypes.ts
@@ -69,10 +69,15 @@ export interface CallReplacesTarget {
 export type MCallBase = {
     call_id: string;
     version: string | number;
+    seq: number;
 }
 
 export type MGroupCallBase = MCallBase & {
     conf_id: string;
+    device_id: string;
+    sender_session_id: string;
+    dest_session_id: string;
+    party_id: string; // Should not need this?
 } 
 
 export type MCallAnswer<Base extends MCallBase> = Base & {
diff --git a/src/matrix/calls/group/Member.ts b/src/matrix/calls/group/Member.ts
index fe2de0af..7404d647 100644
--- a/src/matrix/calls/group/Member.ts
+++ b/src/matrix/calls/group/Member.ts
@@ -33,6 +33,7 @@ export type Options = Omit<PeerCallOptions, "emitUpdate" | "sendSignallingMessag
     confId: string,
     ownUserId: string,
     ownDeviceId: string,
+    sessionId: string,
     hsApi: HomeServerApi,
     encryptDeviceMessage: (userId: string, message: SignallingMessage<MGroupCallBase>, log: ILogItem) => Promise<EncryptedMessage>,
     emitUpdate: (participant: Member, params?: any) => void,
@@ -41,6 +42,7 @@ export type Options = Omit<PeerCallOptions, "emitUpdate" | "sendSignallingMessag
 export class Member {
     private peerCall?: PeerCall;
     private localMedia?: LocalMedia;
+    private destSessionId?: string;
 
     constructor(
         public readonly member: RoomMember,
@@ -140,7 +142,13 @@ export class Member {
             this.peerCall = this._createPeerCall(message.content.call_id);
         }
         if (this.peerCall) {
+            const prevState = this.peerCall.state;
             this.peerCall.handleIncomingSignallingMessage(message, deviceId);
+            //if (prevState !== this.peerCall.state) {
+            if (!this.destSessionId) {
+                this.destSessionId = message.content.sender_session_id;
+            }
+            //}
         } else {
             // TODO: need to buffer events until invite comes?
         }