Dear sir,
We are a VoIP provider and have been in this business since 2006. We build our own software and we've been successful at that. We have deployments based on asterisk that total over 20.000 users. Our setup will allow you to use all servers at the same time and also use realtime sip users with a multi-master mysql setup (not limited just to master-master replication as regular mysql). Servers can be in different geographical locations, they dont need to be in the same colocation.
In addition to all of the above, after this project is finished , we can talk about other features such as fax2mail, mail2fax, web2fax, speech recognition, device provisioning (our provisioning software already supports/provisions 6 different makers of ip phones, including Linksys/Sipura, Cisco, Polycom and Grandstream)
Regards,
Multitel