What you want can be easily done via a TCP Client-Server connection. If you want applications to connect from different IP addresses, ie via WIFI and mobile data without settings in the application itself, then you must include a bridge server with a static IP address that will work as a TCP server to which applications on mobile devices will connect.
Many thanks for your support,
Are you have a way to integrate this library into original walkie-talkie ?
Sure I'll get account from them just after your support.