Can you provide the configs for this setup? You may also check the Class of Service settings to be sure that user is allowed to receive calls. And also make sure that the trunks have that did in the accept statement.
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. If this problem is still happening I would need to see the configurations of both the 7100 and the 3120 along with the output of a debug voice verbose, debug sip stack messages, and debug isdn l2-formatted all enabled at the same time. You can upload those in a ZIP file to our FTP server with the instructions below.
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