Bluetooth device

Forget or unforget?

Should be the former, in Settings>Bluetooth.... that the "i" in blue circle at the right side of the BT accessory, then tap "forget this device".

Should be the latter, you might need to re-pair that BT accessory as if if it were new for your iPhone.

Regards

Giulio

Tap on the (i) and tap forget this device

If it does not have an (i) then reset network settings and restart the phone.