We have 2x SIPGate basic accounts and pay the monthly flat rate for uk mobile and landline calls (£9.95 per month at the time i write this). We have two 01 numbers but we didnt port any across.
Our setup is using a Gigaset N300A and it works great via our fixed wireless internet connection (ADSL and Fibre both non-existent where we live).
At first we just did the basic PAYG type of ting and it worked out cheaper for us to use the flat rate package (you can add or remove the packages as you like).
Ive also connected to one of the 01 numbers via my android phone when abroad and it works just the same.
(Note: I am not affiliated with them at all, just a happy customer)
Moorsweb rural wireless broadband
12Mbit down / 7mbit up, great service.