Hello @Jonathan72, welcome to the community!
I'm really sorry to hear about your phones behavoiur. Is this something the phone started doing recently, or has it been going on for a longer period? Do you hear sounds if you listen to music or similar?
Have you performed the diagnostic test for your phones speaker and microphone? You can find the diagnostics if you go to Settings (> System) >About the phone > Support/Diagnostics. Run the tests and see how the phone behaves outside a call.
I also recommend that you reboot your phone in safe mode, just to make sure that no third party application is interfering with your phones functions. In safe mode, no downloaded applications are available, but you can still use the basic functions like calling, text messaging, accessing the internet and using the camera. To exit safe mode, you restart your phone as usual.
You write that you done a factory reset, did you do the reset in the phone or did you perform a software repair using Xperia Companion? A Software repair goes deeper than a regular factory reset and removes all software from the phone and reinstalls the latest available version. Just make sure that you do a backup using Xperia Companion before you start, so you don't lose anything important.
Keep me updated! I would love to hear how things work out.
