Sometimes the iPhone gets stuck in headset mode and thinks it has regular earphones or ear buds attached even though it does not. Here are a few things to try.

iphone headset socket

With headphone

• Lint may be trapped in the connection. Try blowing into it or try canned air.
• Plug and unplug your iPhone headset a couple of times.

Note: Restoring the iPhone is not a fix!

Personally, I’ve had this problem. I believe it is due to the debris in the socket. Blow some air into the socket and you should be all set.

Cheers
~nevervoid