Creation of mobile application for android with API for 3CX PBX and MySQL

I need to make an Android application with a series of functions and create an API so that the application can work with the 3CX PBX and with our MySQL database on our servers. The pbx is working now, just like the database. The application must have the following functions among others: -Customer registration and login. And register with facebook and Gmail -Google Play payment gateway -A basic control panel for the application, an internal mode, where data such as purchase history per client, balance, number of registered clients, number of clients using the application, location and some other data are collected. -Control panel for the registered customer where you can buy minute packages and see your balance. and access the softphone to make calls. -The application through the API, you have to activate an extension for each registered customer at the time of the first purchase, you have to activate the extension while you have a balance and deactivate the extension when there is no balance. -The application must have an integrated softphone, which does not have to be the 3CX softphone but is compatible with the PBX, and the API must register the corresponding extension's softphone when it is created. -Emission of automatic invoices with business data by service. -Some other function such as referral program, etc. Access to the server is provided through VNC, where part of the platform is already mounted and basic 3cx orientation to manage in the pbx control panel, and assistance whenever possible. We already have a logo and color palette for the graphical interface of the application, but more or less thought of as we want it to be. It will be appreciated that Spanish is spoken