I'm now using voipfone with a transferred in BT number. Everything went smoothly.

The thing to note with voipfone is that the way they are set up, all your numbers by default ring the same 'sip line'. Multiple sip lines are available via what they refer to as 'extensions'. By default keeping the number with them is £2+VAT per month, which is quite cheap if you are mostly using it for incoming calls.