Hopefully someone might be able to help as my head is very sore from banging it against the wall :-)

Configuration/Background info:
We have a Grandstream UCM6102 PBX with a bunch of Grandstream IP Phones.
We have two Open VFX trunks and one 2Talk trunk.
We are using the VFX trunks for incoming and 2talk for outgoing. 
Before moving to the UCM6102 we used AsteriskNow with FreePBX and Cisco phones - all was working fine.

Problem:
Incoming calls lose audio (both directions) when a call is blind transferred to another extension. The call also remains connected (doesn't disconnect/terminate) at the end that doesn't terminate the call.

What i have tested/What i know:
- Attended transfers work perfectly.
- If i make an incoming call with 2talk - blind transfers work.
- If i make an outgoing call with WxC - blind transfers work.
- If i make an incoming call with WxC - blind transfers fail.
- I have tried playing with codecs (setting phone, extensions, and PBX to use PCMA, PCMU, GSM, G.729).

Please let me know if anyone requires more information or has any ideas. I am in contact with Grandstream Support and although they have been extremely helpful have not been able to help me resolve the issue.


Thanks.

Dan