This is one way I have done it but there are others. On the users phone I configured a second line (5555). When SRST had kicked in a call coming into a specific number was sent to the number 5555 and all users with that number on there phone heard the ringing.
Basically each line was a call queue for IPCC Express, but if the Callmanager failed you would lose IPCC as well so this was the option that suited the customer best
SRST config
call-manager-fallback
max-conferences 8 gain -6
ip source-address 10.X.X.X port 2000
max-ephones 42
max-dn 144 dual-line
keepalive 10
no huntstop
alias 1 5920 to 5555 huntstop
alias 2 5921 to 5555 huntstop
alias 3 5922 to 5555 huntstop
time-format 24
date-format dd-mm-yy
Obviously your dial peers will decide how the call is routed to the FXO port
Hope this helps
Paul