You can get call preservation & SRST-like functionality on your MGCP FXS endpoints. Treat it like you would an MGCP PRI which you want call preservation enabled (i.e. ‘call application alternate default’) . Then you also need to have lower preference dial-peers pointing to your SRST gateway via SIP or H323. Just make sure you create dial-peers with ‘service mgcpapp’. if you configure your dialpeer without service mgcpapp it will get matched when you go offhook in normal mode and it will go randomly to CUCM or your SRST gateway.
Why not using H323 for your VG224 gateway?