Ericsson MD110 BC8, with 4 remote lims. one remote location started to have a noise that is only on the phones during internal calls for brief 2-3 seconds. sound like a modem sound, call completes ok, and will happen to both analog and digital. checked power supply ok, checked all cables ok, does not always appear. 3 out of 7 calls. works ok for external calls.?? any ideas???