This is what I found out from TAC: IOS 12.1.5YD1 and 12.1.5YD2 are the only IOS versions that support SRST, which is called SRST Phase I. This phase does not support MGCP, however it supports H323 mode.
You can add the SRST feature and use your router as a gateway by configuring your router under H323 mode, or you can wait for a new IOS release that includes SRST Phase II, that supports MGCP and it is scheduled for November.