How to make the move from ISDN to SIP
Find the right provider, yes, but before that... ISDN is fast becoming a technology of the past. Today's telcos have networks that bypass traditional telephony signalling technologies for IP networks: the hardest thing they do is present a "legacy" connection such as an analogue line or an ISDN connection to a customer, as layering a non-IP service on an IP network is non-trivial at a technical level."