you can try replacing the battery, that would at least take the charging out of the equation. if nothing still, make sure that while it was on storage, it didn’t get wet. you could have had water damage. in that case check on water damage solutions. you may want to try a new screen and digitizer.
Its not what’s up issue, it’s apple. It just stopped working for several apps. Most likely an update changed the choice of mic( sounds like a monopoly to use facetime) and it doesnt work or works some times. Another thing is wether you replaced your screen recently. It could well be the flat connectors that is damaged. Its worth checking, swaping the connectors. At last resort, if you change your screen, you will have new connectors and that should solve your problem! Hope it works for you, leave a message if it did.