The phone can be in headset mode even without one actually connected if the connection pin in the headphone jack is in engagement mode. Inserting and removing a headset from the jack may reset that, or blowing slightly into the jack in case the pin may be stuck down
Try rebooting the phone.
If that doesn't work, it could be the headset jack is broken. Try deloused recommendation.
If that doesn't work, maybe you can use the app SoundAbout from the PlayStore,
What is the make and model of your device? Do you see any signs of physical or water damage on the device?
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