They planned to use the same IMS server as the capacity was huge so why buy two sets. It doesn't matter what the transmission medium is with IP can be Mobile or BB over Fibre or BB over Copper. The call server and SIP/UDP session is just between IP end points, ONT to Call server to set up the session then ONT ( UDP) between IP end points direct. Also ONT to Call server to close down the call and record usage.
Encryption is to prevent fraud and keep the whole system secure. It is also why it won't work over a different hub. ( and why they won't release the SIP server instructions).