Iphone 6- No sound- Intermittent sound-headphones

Hi everyone,

I recently bought an iPhone 6 off a friend. It was out of contract and he was moving abroad.
Anyway, after reseting and installing the latest software, I've noticed a strange reoccurring problem.- NO SOUND.

Basically, whenever I adjust the the volume controls on the side, a screen pops up with volume level and the word 'headphones'
There are no headphones connected, and there is nothing wrong with the headphone socket. I don't think this is a hardware problem, but more a software problem.
The reason I think that is that when I go to the sounds menu in Settings and choose a ringtone, music plays through the speakers, and if I press either of the side volume buttons, the word 'ringer' is displayed instead of headphones. As soon as I exit the ringtone selection menu, it automatically reverts back to 'Headphones'

This problem has happened twice- First time it fixed itself after I opened and closed the FaceTime app (another reason to suggest software issue) Second time it fixed itself after a 20 minute drive. I didn't touch anything....

At the moment the phone is working perfectly, and I've just made a backup.

Has anyone else had this problem and found permanent fix for it?

PS. It's not the silence button on the side 😝