7945 SIP45.9-0-3S--------CallManager 8.0(3a)----SIP Trunk----3845/IOS 151-2.T1-------PRI/PSTN
The below URL states Call Preservation is on by default on SIP Trunks in CallManager.
However in testing SIP SRST I found my call to PSTN dropped when CallManager went down. Maybe I’m missing some commands, but I can’t find a Cisco.com document about SIP Call Preservation with SRST/CallManager IOS 15.
http://www.networkworld.com/community/node/29909