Build me a VOIP-Mobile-Gateway as an Andoid App

I'm looking for someone to develop me an Android app that acts as a ""SIP server / mobile gateway" and makes the cellular line of the smartphone usable with a SIP client. So it is also a kind of VOIP mobile gateway. It may be possible to port the free IP telephony software Asterisk.org to use the cell phone as a cellular gateway. However, it is important that the APP serves both as a gateway and as a SIP server, so you can log on to the app with any VOIP client to make calls via the smartphone's cellular network using SIP protocol (IP telephony).

Important: The app should run on any modern Android smartphone running Android 10 or later.