Hello, As simple as it seems this is a big project and if you check my feedback you will I have done similar projects with FusionPBX / Freeswitch. So I am going to be bias here - recommendation is definitely to go with FusionPBX because I have been using and building systems with FusionPBX for 3 years now and it is a great project. I also am involved with the project and plus Mark Crane and I are great friends. I also hangout on the FusionPBX channel on IRC.
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A single Opensips Server will not give you any redundancy you need to have a minimum of 2 servers. Personally I recommend using Kamailio that way you will benefit from the work that I have already done to integrate FusionPBX and Kamailio and the months of testing that took place.
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Also, just so you are aware FusionPBX and ASTPP will not be able to share the same Freeswitch install. I will be more than happy to discuss in more detail. Best regards Errol Samuels.