Router, T1 Question

Hello,

I have a router running Vyatta.  It doesn't have a T1 card, and they are too expensive.  I was looking on ebay and see Cisco 1721 routers.

I'm wondering if I can terminate a T1 into the 1721 and then connect into the Vyatta router.  However, I want to be able to control everything from the Vyatta.  So basically make the Cisco pass all traffic...

Can anyone explain if this is possible, and how complicated/sloppy it will make things?