I need a VOIP Server for maximum of 1500 simultaneous calls between Android clients and a PSTN network. I installed an Opensips 2.1 server on Ubuntu Server 14.04 but have these problems:
1. Opensips Clients can call each other but without audio transition. It seems that this is a NAT traversal problem. I installed RTPProxy but could not solve the problem.
2. How can I configure this server with PSTN network?
3. How can implement a prepaid billing for my Opensips using ASTPP? I installed ASTPP but could not connect it to Opensips.
I want you to configure my server and learn me how can I do this myself.
The PSTN server is in Iran.