external users cannot dailin conferencing (Network Steve Forum)

external users cannot dailin conferencing

we setup a Lync 2010 meeting, internal is working fine but external phones from pstn cannot dialin to the conference, when type in the meeting ID and hash(#) user dialing will hear: " Sorry I can't seem to connect you to your meeting right now. Try your call again later. Goodbye."

while in windows event viewer I notice below error every time I try dial to the meeting:

"User failed to join the conference.

System.ArgumentException: The string argument cannot be null or zero length.
Parameter name: uri
at Microsoft.Rtc.Signaling.ArgumentVerifier.ThrowOnNullOrEmptyString(String str, String paramName)
at Microsoft.Rtc.Signaling.SignalingUri.CheckSipUriString(String uri, String argumentName, Boolean allowEmpty)
at Microsoft.Rtc.Signaling.RealTimeAddress..ctor(String uri)
at Microsoft.Rtc.Collaboration.Conferencing.ResolveConferenceAsyncResult.GetRequestTarget()
at Microsoft.Rtc.Collaboration.Conferencing.ConferenceAsyncResult`1.StartServiceRequest()
at Microsoft.Rtc.Collaboration.Conferencing.ConferenceAsyncResult`1.Start()
at Microsoft.Rtc.Collaboration.ConferenceManagement.ConferenceServices.BeginGetConferenceUriByPhoneConferenceId(String phoneConferenceId, AsyncCallback userCallback, Object state)
at Microsoft.LiveServer.Caa.CaaCall.BeginGetConferenceUriByPhoneConferenceId()
at Microsoft.LiveServer.Caa.CaaCall.RetryWrapperForBegin(CAAPerfCounterOperation operation, BeginOperationDelegate beginXXX)
Cause: Administration issues.
Resolution:
Verify that Conferencing Attendant is installed in a supported topology and the dependant Front End servers are functioning correctly."

so in short:

LYNC meeting will set up

Internal and external can access in the LYNC call via a computer

Internal and External can text

Internal can be heard by all parties (to both internal and external)

Unsure if external can be heard

Presentation can be seen by all parties (internal and external)

pls help