Pugwash, and anyone else who isn't quite ready to throw away 020 numbers...
There are a number of providers of VOIP (voice over IP) numbers and i am happy with mine, If you get no joy with yours then i suggest you contact them.
Andrews and Arnold ported my 020-8299 number and i got a refund from BT. First negative phone bill ever
Some routers provide support for an old style phone (POTS - Plain old telephone service).