BT Hosted VoIP (Enhanced) should be able to do this (we're currently in the process of setting this up for one of our customers).
The idea is that we will migrate the geographic number (currently with another VoIP operator that has recently gone bust) onto the BT Hosted VoIP (Enhanced) service. We can then set up unconditional call forwarding to the new ISDN30 lines. We're putting the customer on BT One Plan Plus which means there shouldn't be any call charges (One Plan Plus customers receive free intra-company calls which includes BT VoIP calls to BT landlines). If the OPP Fair Usage Policy is exceded on this line (which would be very hard to do) we would then look at implementing one of the Hosted VoIP call packages.
If you move this number to a 3rd party VoIP provider, make sure you obtain written guarantees for call completion and quality and find out how much it will cost you for each forwarded call.
BT Local Business Newcastle
