Got an unusual problem I need help with.

[#1]

Hi,
I have an unusual problem, my phones keep ringing about every 30 minutes but there is no one there.  We have had to disconnect the phone line to stop them ringing.
I am trying to solve the problem and my first port of call is going to be the wall mounted ADSL filter (the one with separate connectors for the phone and ADSL).  This seems to be a logical step as replacement is fairly cheap.
So this leads to my questions,
1.  Am I barking up the wrong tree, is there a more simple explanation to the problem.
2.  I have a good connection with a low SNR and want to keep it that way, what make/brand of filter (Wall mounted) would you suggest.
3.  If the filter is not the problem what else should I be looking at.
Cheers in advance for any info.
Blinky

[#2]

Highly unlikely to be a fault with anything in your house.  I'd suggest plugging a phone into the test socket (remove the lower half of the master socket) and if/when it still randomly rings report it via https://portal.plus.net/wizard/?p=wizard&page=22965&wizard_id=38

[#3]

Does Caller ID or 1471 give any information about a caller? It could simply be an automated system repeated dialling you.
See Silent Calls.
http://www.plus.net/support/phone/nuisance_calls.shtml

[#4]

You may have to report a phone fault here if Plusnet is your phone provider.

[#5]

Cheers for the replies guys,
1471 does not recall any numbers other then the genuine calls that are received.  I will place a fault report through the website and see whether the PN guys can find anything before trying anything else or spending money unnecessarily.
Cheers again guys,
Blinky

[#6]

Cordless / Dect type phone or a wired phone ?
Are the phones that ring on separate sockets ? I.e. one phone plugged into one socket and another plugged into the second ?
I'd second the above about trying a single phone on the test socket to rule out any internal wiring or faulty phone causing the others to ring.