VPN question, related to PRI

We have a main office and two branch offices. We connect to the branch offices using Point to Point T1’s. We are using PRI T1 for our phone service, manages by a Nortel BCM400. When we need to phone the branch office we use a 4 number extension.

Now, if we switch over to SDSL or SHDSL or even just Internet T1’s for each location and then VPN everyone together, can we still just use 4 number extension to communicate over the PRI phones?

I was told by someone that PRI only works over Point to Point T1’s.

Thank You