Have you migrated phones from PRI to SIP?

We're a medium-sized financial institution and have a reasonably sized call center component to our operations that we handle in-house.

We have approximately 50 lines available from our local telco via a PRI. We are not on great terms with our telco, and are researching migrating our external lines to SIP into our phone system - not only to drop their service but also intending to reduce costs in the meantime.

Have you completed a similar migration? What SIP providers are you happy with or should we stay away from? Any gotchas or things we should be on the lookout for in the process?