Have just found this documentation so that helps some… Still not sure if user status will change when staff are on calls etc… between servers. I will setup two test servers and experiment.
as long as the users have subscriptions and you are using different domain name for both servers this should work fine.
Anyhow it more be a better idea to use only one central server as you may already have a VPN connection between your offices. This should also allow you to use only one VoIP server, making the management more easy.