If you want to go with more IP phones and get away from the PRI or have another device that changes the PRI to SIP for the conversion then you can look at Grandstream and also to have analog phones that can either be on the processor or when you need more than 4 or 8 (can't remember the exact number).
We work with that platform as well but not with PRI's as that is a real headache to setup initially however once installed and set up works well.
The best portion on that is that the phone programming is in the cloud and the phones grab it by themselves as long as they have Internet access.
There are also other brands out there that do the same but they are mostly designed for IP solutions with SIP trunks, not PRI's
Joe
FHandw, ACSS, ACIS