There is often a debate for which protocol to choose MGCP or H.323? Although it is possible to configure both on the same gateway, this is strictly for MGCP fallback to SRST.
The key aspects is that with H.323 compared to MGCP, H.323 requires more configuration on the gateway since the gateway must maintain the dial plan and route patterns.
CallManager only sees the router as one gateway. Calls are sent to the gateway but CallManager cannot specify which port the call is sent to.
The CallManager does not even know that multiple ports exist on the gateway
With MGCP however, Cisco CallManager knows and controls the state of each individual port on the gateway.
MGCP allows complete control of the dial plan from Cisco CallManager, and gives the CallManager per-port control of connections to the PSTN.
For these reasons call control requires MGCP or H.323.
Rgds
Allan.