I have a Fritz!Box 7390 modem router which has a DECT base station built in. One of the minor bugs with this particular box is that about once a week I get a phantom telephone call in the early hours which occasionally results in an internet disconnect. I have also had the situation that you describe: i.e., the internet disconnects with an incoming outside call. Many people have raised this with AVM the F!B manufacturer. Their response is as follows:
Ghost calls and DSL disconnects parallel to incoming telephone calls occur
due to electrical impulses. The latter mostly is caused by a wrong cabling
with an analog landline.
Verify that only the DSL splitter is attached to the main telephone socket
and that no other terminal devices (e.g. telephone, fax machines etc.) and
no other telephone sockets (e.g. via extension leads) are connected to the
main socket. Connect any terminal devices to the extensions of the
Please find our information on the "ghost calls" by clicking on the
All that said, I have used double-filtering with some success when I was on an ADSL line. It is important to remember to connect the modem to the first filter and the telephone line to the second.
And finally, I thought that I read something a couple of years ago about BTOR rolling out integrated high resistance fault testing. Did anything come of this as I recall that the success rate was in excess of 90%?