The problem has been isolated to the remote modem, or the remote system or controller.

Local System
 .----------.                                               
 |          |                                                
 |          |                                                Remote System
 |          |    Local Modem                Remote Modem     or Controller
 |          |    .-----------.              .------ ----.    .----------.
 |          +----+           +------//------+           +----+          |
 |          |    '-----------'              '------ ----'    |          |
 |          |                                       ^        '----------'
 |          |                                       |                   ^
 |          |                                       |                   |
 |          |                                       |                   |
 '----------'                                        The problem is here.