Look on the bottom of the phone, on the left of the charging port there is a black hole/dot.this is the microphone. Is there any foreign object obstructing it? If so remove it. If not then unfortunately it must be damaged, I would recommend taking it to a local repair shop to get it diagnosed.
I was curious myself and I tried looking for new techniques online. Couldn’t find any. Finally about a week of exploring the interface I found the solution!!!! Hold down the home button to trigger Siri. Raise it up to your ear and ask a question. She answers using the ear speaker! This will also test the proximity sensor. Ps: You’re welcome!
Sorry buddy, logic board. It’s the beginning signs of touch disease. You could microsolder a new chip but it’s much too complicated. Edit: like puffs commented, you could shop around town to get someone to do it for you. If you can get a good price go ahead. But keep in mind a used 6 only sells for around 80-100 nowadays.
The iPhone 5 had a similar issue. It was only solved when you do a reset through dfu. After doing so it would give a no sim error. After removing and reinserting the sim it would work normally. I’m not quite sure if you would want to try this with the SE. but it is a major headache. Sometimes apple gets on my nerves.
Replace the proximity sensor cable. Then do a full restore via iTunes. It’s a hardware issue that caused a software glitch. Also try using the lcd shield from your old screen if you’re not already as aftermarket ones cause problems.