The easiest answer to this is to first re-set your phone.

How to Turn Off iPhone - How to Restart iPhone - Restarting iPhone - Rebooting iPhone

If that doesn't solve the problem plug your phone into the computer
Open iTunes to discover the iPhone.
Choose your iPhone and when the iPhone page opens
choose restore. This should backup your iPhone and
restore default settings. Then your iPhone should re-install
all your previous information and settings including
previous bluetooth pairings. Takes a bit of time but
usually solves most iPhone issues.

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