Further to bokbergs question it is not entirely clear as to the context of where the modems are located and to which equipment you wish to connect to .”

The L2TP protocol runs on most mutipurpose routers and enables a mobility solution for tunneling PPP over IP and as you have VPN should be there already .

Using L2TP tunneling, an Internet Service Provider (ISP), or other access service, can create a virtual tunnel to link customer’s remote sites or remote users with corporate home networks. The L2TP access concentrator (LAC) located at the ISP’s point of presence (POP) exchanges PPP messages with remote users and communicates by way of L2TP requests and responses with the customer’s L2TP network server (LNS) to set up tunnels.

L2TP supports multiple protocols and unregistered and privately administered IP addresses over the Internet. This allows the existing access infastructure, such as the Internet, modems, access servers, and ISDN terminal adapters (TAs), to be used

Again this would depend on the context of what you are trying to achieve and the existing technical framework .
Check out :
http://www.cisco.com/univercd/cc/td/doc/product/software/ios120/120newft/120t/120t1/l2tpt.htm#wp5940