The time capsule does not have a modem built in, what sort of cable are you using to connect it to the phone line?
Your original modem/router will work fine with the time capsule but you need to get only one of them to do the routing part.
To make life easy just get a new modem like this
http://www.ebuyer.com/product/118668...edium=products
This is the same setup as I've got and it's been working flawlessly for 3 years now.