I only have the one master socket, so yes it is filtered. I've tried three different filters and also tried running two filters in series. No luck! Apparently my cordless DECT phone could be causing interference so I'm going to try a 'normal' phone.
I think you have covered the filtering then! Sounds like you know what you are doing. I'd be suprised if the DECT was the issue, but I guess it could.
Couple more silly Q's, but ...
1. have you unplugged the DECT phone and rung your phone number to see if that drops the ADSL connection.
2. Also are you using an internal ADSL router (in the computer) or an external router? Any chance of beg, borrowing or stealing (anything but pay for) somebodies spare to see if a different brand would cope with whatever the line problem is better. Unfortunately I gave my spare away last year.
3. Did this setup work in your old house?
4. Have you concidered cable!
Ideally want to have a standalone phone so that we don't have to have the PC on to make/receive calls.
Haven't got a clue about providers because I've not really researched it much.
There are a lot more standalone phones on the market than when I last looked. We used Snom for testing at my last place of work, bit pricey but there was not much choice then. Now you can get fancy dan cordless affairs that look much the same as a regular phone.
I guess the first question is do you want to use a standards based protocol (e.g. SIP) or a propriatry protocol (e.g. Skye). A SIP phone will allow you to move freely between provider that support SIP (e.g. sipgate as mentioned above). A Skype phone will mean you are stuck using Skype (which may be just fine).
I guess it comes down to cost, reliability and range of services. My concern would be that you still have to pay BT line rental+internet provider+whatever VoiP phone/call costs - which may be cheaper than you pay now I guess.
Erm, best of luck! Just realised I have waffled on without being helpful. I'm kind of interested in how you get on should you make the switch to VoiP.