Cisco Call Manager Voice Quality issue

[4Riel]

I am working with a Vendor who has a Cisco Call manager and am trying to find out where the problem lies. The end phone user experiences problems that are similar to full duplex and half duplex. Where the caller A on site, is talking to caller B and caller A's voice goes over the top of caller B voice is diminished in volume and sometimes completely cancels it out. Similar to what you hear on a cell phone. The Vendor has set it to full duplex so I am at a loss.

Has this ever happened to anyone else, and/or do you have any advice on where I can look next?

One last thing, this is a normal TDM circuit being delived, not a ATM.

Thanks in advance for your help!!!