In SRST third party sip phones I think are not supported. However, you can configure Fall back to CAll Manager Express and get you SIP Phone support. I think support for SIP Phones was available with CME 4.2 and above. So if the phone can support multiple connections from top down, you can set the CME as the third server to connect to when the first two or first one is not available.
Someone else may have a better idea, but this is just my humble opinion.
SIP SRST Admininstration Guide states the following:
Cisco Unified SIP SRST can support SIP phones with standard RFC 3261 feature support locally and across SIP WAN networks.
So I think that it is possible but theoretically. If the phone support dual registration with CCM and any backup registar server then it works. And it doesn't matter whether this registrar is CME or SRST.
CME and SRST is pretting much the same thing. If you're running the later version of SRST 4.2, then yes it is supported since it is support in CME. I was wrong on the first part of my posting. Thanks on the update privateer.