Sebastine,

If you need to use the centralized call processing model with Cisco Callmanager, you also need the SRST router into the branch offices.

See below at SRND guide:

Cisco Unified Communications SRND Based on Cisco Unified Communications Manager 6.x - Unified Communications Deployment Models [Cisco Unified Communications Manager (CallManager)] - Cisco Systems - Multisite WAN with Centralized Call Processing:

http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/srnd/6x/models.html#wp1043616

When WAN goes down, phones register with the SRST and now and the calls goes to the PSTN

If you don't need SRST you also can deploy Cisco CallManager Express at each branch office, but there's call handling at each branch office.

Cisco Unified CallManager Express Solution Reference Network Design Guide - Cisco Systems:

http://www.cisco.com/en/US/docs/voice_ip_comm/cucme/srnd/design/guide/cmesrnd.html

Hope this helps,

Vitor

if they do not want SRST (which is not best practice) u can configure their router etheir as H323 gateway(with h323 u need to configure dial-peers) or mgcp gateway (registered with ur CCM)

and make dhcp on the router for the voice network and make the ip address on the dhcp option 150 the ip address of your callmanager

if helpfull rate