1: Setup an api, to send a request from magento to a2billing, when a client are singing in Magento. So with this, a client are going to get an account in magento, and in the same time a2billing are creating a SIP/IAX2 account, and send an e-mail with that data.
2: Costumer can add more credit, using magento shopping cart, and checkout options, and notify that to a2billing by addiding that purchased credit.
3: Freelancer should be knowledgeable on Magento and A2Billing for developing the API key
A2Billing is free and open source software for Asterisk