The NM is a network module which takes up a single slot in the router and may, depending on the module, support 1 or 2 WIC's (WAN Interface Cards) - a WIC is an optional card supported in certain network modules (NM).
If you need PRI support you will require a NM, there is not currently a PRI WIC, therefore you would need at least a 3640 to support 3 x PRI.
the follwoing URL outlines the WIC - HW compatibility:
http://www.cisco.com/warp/customer/471/wics.html
