Hello Jason,
You can use the "WIC-1DSU-56K=" card for this on the Cisco2821.
Configuration would be something similar to this:
Router>en
Router#conf t
Router(config)#int s0
Router(config-if)#service-module 56k network-type dds {dds | switched}
Router(config-if)#service-module 56k clock rate 56 {auto | 56 | 64}
Router(config-if)#service-module 56k clock source line {line | internal}
Router(config-if)#service-module 56k data-coding scrambled {normal | scrambled}
Router(config-if)#service-module 56k switched-carrier att {att | sprint | other}
