@westi, in most metro areas in the US the major (CLEC) carriers deliver all new service as SIP they will then dump that into a router/IAD and deliver it as analog or T1/PRI. I can probably count the number of times I've done a traditional/true PRI in the past year on one hand. Everythig else is VoIP with digital or analog handoff. These guys will deliver a dedicated IP only T1 or HDSL to save copper pairs.
@LRB45 if you are looking for an ITSP (SIP only, you provide the Internet connection) It doesn't really matter who you go with. I've had very good luck with Flowroute.
If you are looking for a traditional carrier, the "big" SIP providers to check are
XO
Paetec
Cbeyond
We probably do 30-40% of our new installs as 100% SIP without issue. I'm not quite sure why people see the benefit in SIP from a traditional carrier other than they can get more channels out of a single circuit than with a PRI but few order more than 23 anyway.
Kyle Holladay
ACA-I, ACA Call Center, ACS-I, ACS-M, TIA-CTP, MCP/MCTS Exchange 2007
ACE Implement: IP Office
"Thinking is the hardest work there is, which is the probable reason why so few engage in it." - Henry Ford