From a TAC engineer's perspective, a problem that is 100% defined is 50% resolved ...
So here is what we now know :
You have a 3rd party voip device plugged into a Cat6K switch. The switch has an optical connection to a remote site that has a another switch that connects to a MGCP Call Agent.
I don't believe there is any particular Cisco voice blade installed in the Cat6K, so we are not talking about a Cisco specific voice issue here.
Based on the stats from the Adtran device, you are seeing TX and RX MGCP control traffic so you have an open session to the call agent. This means you have IP connectivity on UDP port 2427.
Since the RTP RX counter is zero, then it is apparent no media is getting to the IAD.
And here is what we don't know :
Is the entire network layer 2 switched network (no routing) or is there layer 3 routing ?
Is there any packet filtering being done on the CatK ?
Are the RTP media flows actually getting to Cat6K in the first place ?
How many connections does the Cat6K have the rest of the network?
Before jumping to conclusions about possible reasons such as access lists, you need to follow the RTP media path from the source in a logical approach to determine where it is failing.
Assuming the Optical connection is the only ingress point into the Cat6K, do a SPAN session on this port to another port that has a PC running wireshark, and make a test call.
Need help doing the SPAN -
http://www.cisco.com/en/US/products/hw/switches/ps708/products_tech_note09186a008015c612.shtml
Get a packet capture and confirm if you have RTP packets coming into the switch in the first place. If you do see RTP, then you can concentrate on your Cat6K switch.
If you don't see any RTP, then do the SPAN or similar off the Allied switch and see if it is getting this far.
Follow the RTP back to the source and you will find your answer.
