Depends on what your budget is. You can get a Digium IP->PRI gateway box that will plug into your PBX’s T1 port for about $1100. Their gateway is just basically Asterisk on an “embedded” platform with dedicated hardware. Or you could just grab a PC in the office, stick a $500 single port Digium T1 card in it and run Asterisk on it.
The other, more esoteric solution, is to find a Cisco AS5300 with two T1 ports on it and you can use one port to terminate the IP side of your point to point T1 and the other port to connect your PBX. It is an odd ball solution and really only needed if you need a router for your PTP T1 as well. But if you do find one cheap enough you can use it without using it to terminate your PTP T1.