[Serusers] Setup Architecture Question

I am working on a plan to implement VoIP at a company which has 3-4 remote sites across the state connected back to the main headquarters. I would like to use SER for its scalability and feature set but I am unsure if it will meet some of my business needs. I need to have a single SIP domain across the 4 sites while at the same time ensure remote survivability and access to their local PSTN connections if the WAN connection to the main headquarters goes down. I also want to use the WAN links and PSTN connections at each site for Toll Bypass within the state. Will SER accommodate this type of architecture? Thanks, Jeremy