Were you ever able to resolve this issue? If so please come back to this post to provide an update so others can benefit from the solution. Typically audio drops after a set amount of time are related to SIP keepalives or firewall timeout issues. You may want to investigate if either of those are at play. My next suggestion would be to upgrade to the latest version of firmware listed as the Extended Maintenance Release from the NetVanta 7100 firmware page. There will be a link for the release notes next to it that you should reference to make sure you have the required version of phone firmware loaded for that version of chassis firmware. If you still have problems after that I would suggest collecting the output from a debug voice verbose, debug sip stack messages, and debug isdn l2-formatted (if using PRI) all enabled at the same time while you recreate the issue. Then you can submit those along with a copy of the current running-configuration to to the FTP server with the instructions below and I would be happy to take a look.
Open Internet Explorer web browser on their PC
Type the following URL: ftp://ftp.adtran.com
Press the Alt key, click View, and then click Open FTP Site in Windows Explorer
Double-click the "Incoming" folder
Drag and drop files from PC into the Internet Explorer window
Reply to this post with the exact filenames used so we can retrieve the files
Include a detailed description of the call flow