One Way Audio After Hold

All,

I am hoping that someone may be able to point me in the right direction. We are having an issue with our current VoIP infrastructure and I am trying to narrow down where the issue is coming from (Carrier, SBC, PBX, Network)

Issue:

Employee calls outside phone number and far end answers. Both hear each other talk fine. The far end puts the call on hold and transfers it to an auto-attendant and the audio drops on the call. The issue seems to happen sporadic to different out-bound numbers but I am able to replicate it each time to the same numbers.