VOIP and SIP Solution

Hello, I am looking for a solution for a startup I am planning in Africa. Normal telephone calls are done using PSTN. I am designing a phone dialer or softphone. What I want to achieve is when a caller (PhoneA) initiates a call to a receiver (PhoneB) using my own phone dialer app, the call is routed to a server using VOIP.. There are transactions done in the server. Basically, it is one-directional from the server to the subscriber placing the call, but it could be two-ways with numerical input only from PhoneA to My Server by server prompt. Once the transaction is done in the server, I want to achieve one of these: 1. My Server returns control to phone dialer (PhoneA) which places the call using the regular phone network (PSTN) or 2. Call is made / connected from my server via SIP solution to PhoneB without using the regular PSTN The target is for the app to be used by millions of subscribers in placing calls. So the solutions have to put that into consideration. The app is gong to be designed and built by me. If you can do it, we can discuss on that as well. I explained the whole operation because I would like to get some insight (or input) on how to go about API and other integrations in designing and building the app to tie into VOIP Please let me know if you have any questions. We can discuss price. Do not worry about figure in the Set Fixed Price. Thank you. Ekene