Outgoing calls disconnects using new SIP trunk

Question

Hi,
i have a lync 2010 setup. all calls (until now) are going through isdn lines. a few days ago i have configured a sip trunk to use. i have configured new dial plans and voice routes for this.
the issue is that when i call an external number the call is not established. if i click redial usually the call is connected fine. incoming calls are working just fine

Answer 1

i got this from the logging Tool:
TL_ERROR(TF_PROTOCOL) [1]070C.083C::10/02/2014-09:53:35.314.000001a9 (MediationServer,GatewayCall.GatewayParticipateComplete:gatewaycall.cs(1740))(0000000002446616)$$START-MEDIATIONSERVER
MediationCall: 3ff4fd37c1dc4083bc2a12218476c925
CallId: 005e2a14-9630-4f2e-b9ad-9f22d8d67c5d
From: sip:[email protected];user=phone
To: sip:[email protected];user=phone
Direction: Outbound
Start-Line: FailureResponseException: ResponseCode=503 ResponseText=Service unavailable
Microsoft.Rtc.Signaling.FailureResponseException:A 503 (Service unavailable) response was received from the network and the operation failed. See the exception details for more information.
   at Microsoft.Rtc.Signaling.SipAsyncResult2`1.ThrowIfFailed()
   at Microsoft.Rtc.Signaling.SipAsyncResultBase2.EndAsyncOperation[TResult](Object owner, IAsyncResult result)
   at Microsoft.Rtc.Signaling.SignalingSession.EndParticipate(IAsyncResult result)
   at Microsoft.RTC.MediationServerCore.GatewayCall.GatewayParticipateComplete(IAsyncResult ar)
Detected at System.Environment.get_StackTrace()
   at Microsoft.Rtc.Signaling.FailureResponseException..ctor(String message, Exception innerException, SipResponseData responseData)
   at Microsoft.Rtc.Signaling.RealTimeException.GetWrappedRealTimeException(String message, SipResponseData responseData, Exception exceptionToWrap)
   at Microsoft.Rtc.Signaling.SipTransactionAsyncResult`1.ProcessFailureResponse(SipResponse response)
   at Microsoft.Rtc.Signaling.SipInviteAsyncResult.ProcessFailureResponse(Object state)
   at Microsoft.Rtc.Signaling.QueueWorkItemState.ExecuteWrappedMethod(WaitCallback method, Object state)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Threading._ThreadPoolWaitCallback.PerformWaitCallbackInternal(_ThreadPoolWaitCallback tpWaitCallBack)
   at System.Threading._ThreadPoolWaitCallback.PerformWaitCallback(Object state)
i get the 503 service unavailable... which i find strange. because first outbound call ALLWAYS fail and the second always succeds