custom sip client

we need to build a custom sip client that connects to a proxy by passing proxy blockages from certain countries that use such sip closing methods. also it must run in the background and have normal functionalities like connecting to the phone book of the phone and having speaking and three way calling. this needs to be automated where customers can sign up without having a phone number but if they choose to have a premium account and get a phone number, they can choose from a list of DIDs that we have (this needs to have the app be synchronized with our database of DIDs). this should happen without human assistance on our part, billing should be automated for premium accounts with monthly withdrawals. other details can be discussed when bids submitted and subject to approval