SRST alike solution with *

I’m looking for a concept to implement a Cisco SRST alike solution based on *.

Our current solution is based on independent * at every site (main and branches) interconnected with iax2 trunks and the respective routing. At every branch site there are local PSTN-trunks used for local connections.

What i’m looking for is some solution that gives me a branch “surviveability” but a centralized user administration (with centralized CDRs etc…). As it is in a Cisco SRST solution aswell call feature limitations during wan outages wouldn’t matter. Another important point are media-flows, should be local to the branch, for inter-branch calls or offnet calls that leave via a brunch pstn-trunk.

Has anyboy done something similar? Any ideas?