You can setup a second dialpeer with same number and a higher preference. This dial peer can then run sip to the voice gateway(SRST router). SRST router also needs to be setup with sip to receive that call under srst mode.
SIP and H323 gateways dont register to a router like MGCP/SCCP does.
Sankar Nair
UC Solutions Architect
Pacific Northwest | CDW
CCIE Collaboration #17135 Emeritus