NEC Topaz - No ring problem

[balcomm]

I have installed a Topaz in a Winery about 10k from the local exchange. Intermittently (regularly) they don't receive the incoming ring on the handset, but a tone ringer across the line rings as normal.

The system works fine in my office. I thought the line voltage may be a problem but they test fine, although the lines are supplied on a 4 circuit digital unit.

I replaced the system, still the same problem. Has anyone else experienced this issue?

Any ideas on solving would be appreciated.

Noel