Mark,
When you have a centralized CM deployment, your remote sites may be deployed with either SRST or CME. The choice of which one to use depends on how much bandwidth you have available in the wan to support the signalling traffic for the X number of phones you have out in the remote sites. With CME, signalling traffic remains local to the lan at each remote site and each remote will have to be managed on its own. In both scenarios, interoffice calls do consume bandwidth across the wan.
Off late most of my centralized deployments with remote sites are deployed in a hybrid mode. Its called CME with SRST fallback support. Its pretty cool, because you can have all CME like features at the remote site, when wan fails and phones failover to the router at the remote site. You can setup BACD, hunt groups, pickup, call park etc.. This is not available with traditional SRST. Check this link for more details.
http://www.cisco.com/en/US/products/sw/voicesw/ps4625/products_configuration_guide_chapter09186a00807e0566.html
HTH
Sankar
PS: please remember to rate posts!
Sankar Nair
UC Solutions Architect
Pacific Northwest | CDW
CCIE Collaboration #17135 Emeritus
