I’m wondering if you could do this with squid proxy. It’s been ages since I used it but I recall the ability to write PHP apps for it that “mangle” connections.

I’m wondering if this is a bit of a xy problem.

Because there’s tools like Fiddler and Hoppscotch that come to mind but it’s a bit outside your scope.

I haven’t played with third party dialers since gingerbread was new.

But, have you disabled any power saving features both from the manufacturer and the OS?

I wonder if something like LineageOS would be suitable for you?

I’m not super familiar with the subject but it’ll probably be something based on Tesseract.

Maybe try gImageReader.

My clients when they text me the server is down.