Build a cellular GSM callback system to allow unlimited outbound calls

Hi! I want to build an unlimited outboung calls callback system similar to many websites I have already seen on the web, but improoved. This solution must work for any phone (landline or mobile and not just smartphones), so we can NOT use just apps to make the calls. Steps to make a call: 1 - Call our access number to get a busy signal and hangup. 2 - Our system detects your caller ID and calls you back in a few seconds to give you a dial tone. 3 - Enter the phone number you want to call. 4 - The system automatically connects you with your called party. In my country, we have carrier plans where calls placed to another phone from the same carrier are 100% free. We can use a GSM gateway to place the calls using the best SIM card for each call (the same carrier from the dialer and other same of destination). Then we connect the 2 calls using a VoIP software integrated with the GSM gateway. I will charge a low flat monthly price for my customers, so I need a customer database to identify the origin of the call (if who is dialing to my access number is an active subscriber). I will need consulting on choosing the best cost-benefit hardware for the GSM gateway and help on the implementation (choosing the best software, etc.). All operation must be very efficient and automated to that the product can scale. Any questions, feel free to ask. Thanks!