You can use many Cisco routers, such as 1700, 2600, 2800, 3600, 3700, 3800, 7200, and 7500 series models, as voice gateways. Voice gateway routers can contain analog ports such as FXO and FXS, and digital ports such as E1 and T1. Most can use the MGCP, SCCP, SIP, and H.323 protocols.

Taken from "Cisco Voice Gateways and Gatekeepers"

Also check this link:

http://www.cisco.com/en/US/docs/ios/12_3t/12_3t8/feature/guide/1700crl9.html