T1/SIP Provider -or- T1 Provider + SIP Provider, experiences?

What other recommended providers are out there such as cBeyond or XO that offers BOTH T1/PRI and SIP connection? (to ensure quality of service)

cBeyond's SIP-Connect (while not in our area), I thought was expensive @ ~$495 for 6x lines, 1500min, $.06/min ($40/add'l line)

I'm TRYING to get a high quality Internet connection that I can run completely w/ SIP connections (no analog, no data). This way I can save $$$ w/ various carriers on unlimited long distance, good int'l rates, etc. I'd prefer that I use my own SIP providers (for both redundancy & savings), but I've read in a couple places to make sure that the T1 provider IS the SIP provider for QoS?

My initial research tells me 'what for?' and mayse well just get a T1 @ these rates, but alas the search continues...

P.S.
Would you feel comfortable running services such as Teliax/TelaSIP/etc over a standard IP-T1 w/ them of course NOT being the provider?

Much appreciated!
- J

::updated::
Anyone have any references I could have a potential customer actually call for this? (Got a sales person here who keeps pushing my buttons, I would GREATLY appreciate it)