Windows (any variety) and "intermittent problems" are synonymous. No help here.
The solutions increase exponentially from this point.
The first thing to check is your Telco wiring. A loose wire could cause the problem. Here in the States most Central Offices (CO) have a "Quiet Line" devoted to providing nothing but silence. Telco's will not provide these special test numbers to customers so you're on your own to find one in your area. We also have a dial-up number that shunts (shorts) the pair at the CO allowing an analog Ohm-meter to measure the resistance of the loop. The form is generally NXX-99xx, where xx is a variety of test sequences. A simple but effective test is to simply grab a dial-tone and click the switch-hook once. You then have about thirty seconds to listen for "noise" on the line. This is step 1. Assuming that you have isolated, i.e. cut, your premise wiring, any noise would be the responsibility of the Local Exchange Carrier (LEC.)
It is helpful to know the location of your CO and the route the cable travels to your premise. Mine travels three miles underground along a busy road with a lot of lorry's (trucks) carrying steel. Prime real estate for inducing a magnetic field into the telephonewiring.
Answering machines, cordless phones, caller ID boxes, and mixed telephones complicate the equation. The "guys with ties" told me that their 50K$ HP box suggested that my equipment added five miles of wire to my loop. I did not disagree and asked them to duplicate the test. The results were slightly different but they still pointed the finger at me. I pointed to the disconnected premise wiring and casually mentioned that their interface was the only factor in the equation. Withintwo months, they installed new plant (outside wiring.)
