Converting Key System to VOIP

We have offices in NJ, MN, VA, DC & CA that we want to be able to connect to via VOIP. MN & VA/DC have PBX in place, while NJ has a Key System in place that is being serviced by a vendor.

Our plan is connecting each site’s PBX, via ISDN PRI, to a Cisco 2600 router with a VoIP module.
The problem site is NJ (and CA).
Can NJ’s (SPRINT) Key System be connected using a PRI line?

We could also implement a PBX that is not being used, for NJ. However I think we’d have to order new circuits (DID & CO trunks), right?

I’ve also heard about IP-Centrex.
Is that a possible alternative?

Which is the best solution?
Any other ideas?