The problem has been isolated to the telephone line equipment that links the local and the remote equipment.

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