My iphone is stuck on headphones, even though the headphones aren't in the phone.
Try quickly inserting & removing your headphones multiple times and blowing on the headphone jack with a can of compressed air.
I know it sounds weird, but seal your lips around the headphone jack the best you can and suck multiple times. It worked for me almost immediately. It's worked for others too. I tried everything before that and it did not resolve it.
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