Normally, a Media Gateway Control Protocol (MGCP) router can get its configuration from Cisco CallManager. This case has a 3660 router with High Density Analog Voice Network Modules (NM-HDA) and 4-Port Foreign Exchange Station (FXS) cards. The Cisco CallManager runs version 3.3(3).
Sometimes, instead of having the router maintain the startup configuration, it is desirable to have Cisco CallManager take care of the details of configuring the MGCP gateway. In such a configuration, you have more centralized control over your devices.
In this case, the MGCP connection between the router and Cisco CallManager has proven troublesome. Specifically, after installing a fax machine to the router, the router fails to respond to the Cisco CallManager requests.
Cisco has minimum memory requirements to run the software on the Cisco IOS Software page for routers. Check the network setup, and after installing drivers and routing tables, make sure there is enough memory. The Memory Calculator tool can verify that you have enough memory for all the devices. After an upgrade from 64 MB to 128 MB, the router had enough memory.
For more information on configuring an MGCP gateway in Cisco CallManager and gateway memory planning, refer to these documents: