diff --git a/TestPhone.st b/TestPhone.st
index e823a0c..86db28c 100644
--- a/TestPhone.st
+++ b/TestPhone.st
@@ -1,7 +1,7 @@
 PackageLoader fileInPackage: 'OsmoNetwork'.
 
 Object subclass: IPAConnection [
-    | socket demuxer queue muxer dispatcher sccp ipa |
+    | socket demuxer queue muxer dispatcher sccp ipa sem |
     IPAConnection class >> initWith: anAddr port: aPort token: aToken [
         ^ (self new)
             socket: (Sockets.Socket remote: anAddr port: aPort);
@@ -14,6 +14,8 @@ Object subclass: IPAConnection [
     ]
 
     setup: aToken [
+        sem := Semaphore forMutualExclusion.
+
         demuxer := Osmo.IPADemuxer initOn: socket.
         queue := SharedQueue new.
         muxer := Osmo.IPAMuxer initOn: queue.
@@ -43,12 +45,19 @@ Object subclass: IPAConnection [
     ]
 
     drainSendQueue [
-        [queue isEmpty] whileFalse: [
-            | msg |
-            msg := queue next.
-            socket nextPutAllFlush: msg.
+        sem critical: [
+            [queue isEmpty] whileFalse: [
+                | msg |
+                msg := queue next.
+                socket nextPutAllFlush: msg.
+            ]
         ]
     ]
+
+    send: aMsg with: aType [
+        muxer nextPut: aMsg with: aType.
+        self drainSendQueue.
+    ]
 ]
 
 Object subclass: IPAConfig [