you may purchase a vpn concentrator for use at the corporate site and have cable modems, or dsl on a ethernet port on your 2524's. it would work like this
RTR 2524 > FRAME > CORP (normal)
RTR 2524 > CABLE/DSL RTR ETHERNET CONNECTION >
CORP..
this is a valid solution which gives you far more bandwidth than ISDN, for about $40 per month, per site. There is an initial cost of $4000 for the VPN 3005 with smartnet. Of course I am assuming you have
a free ethernet port on your routers ? if not you will have an added expense.