is your phone line plugged into the filter properly.....the dog half-pulled mine out once. I'd just try unplugging the filter.......then unplug and put back in the phone line and re-plug in filter.
before you report the problem to your service provider unplugg all equipment (sky box / routers etc) and all extensions then plug a phone that you know works (maybe try in a neighbours / friends line to double check) if your line is still not working then report it. If an engineer proves that the fault is on your equipment / extensions you'll get a bill for appx £150.
I had the problem in reverse; the phone worked fine, my internet didn't. My ISP did some checks and said, rather unconvincingly, that it wasn't them and suggested I spoke to BT to get the line checked. This I did and BT phoned me back (they'll also happily phone your mobile) within 10 minutes to say there was a fault on the line. They then kept me informed, by text messasges, as to the status of the repair of the fault.
Thank you all for your replies. I've not marked one as Best Answer as you all took the trouble to help me. Like Malcolm Y, my problem was resolved in a similar way. There was a fault on the line betwen my house and the local exchange. BT kept me informed by text and the following day I saw a BT OpenReach van just down the road from me. I went across to it and there was the poor BT engineer covered in snow working in a half flooded inspection pit. Although it was late in the day and getting dark he just managed to get my line up and running by 5:00pm. On this ocassion I've got to say 'Well done BT'.