The 2811 does not really come with any interfaces. In you case you could use the Aux port to attach to this modem. Its been a while since I did this but I assume it still works on a 2800.
To use interface cards you would need something like a wic-2t a/s or if you really want you could completely replace the modem and plug the isdn line into a wic-1b-u card. If you are going to pursue cards make sure you get the correct ones some of the older ones are not supported in the 2800.
More than likely this is a PPP connection so you will need all the options from your isp to configure this