hello Ekene, I'm a former VoIP technician, I worked 10 years on the NOC department for a US company; I read your requirements and your image scheme, I understand your want to build your own app in such a way that your subscriber sends the call from app to Asterisk, then asterisk computes whatever value you need and it either allows the originating party to dial out from the PSTN trunk or it will dial through the VoIP trunk, am I correct ? Please feel free to contact me so we can go through more details and I probably can help your design this app. I'm not an Android developer though, but if you're the one that will build the app I can probably point you to the correct place, and offer you the best solutions.
Thank you.