That depends on whether that T1 terminates onto an external CSU/DSU or not. If the line comes into a jack or a bisquit box, then you'll need to run a cable to your router and purchase an internal CSU/DSU card for the router (WIC-1DSU-T1-V2 for the 1841), or buy an external CSU/DSU, and then run serial to the router and (still have to) buy a serial WIC (like WIC-1T). Internal CSU/DSU are preffered, since you can get more info about the status of the line through the router's command line interface.