Please try this first on your iPhone, follow these steps>
Tap Settings. Tap General. Scroll down to the bottom of the page and tap Reset>Tap Reset Network Settings. You will get a prompt warning you that this option will delete all network settings and return them to factory default settings. Tap Reset to continue. Wait while your iPhone resets its network settings.
Once the reset process is done, your network settings will return to its factory default settings. Your iPhone will also automatically restart. Then remove the iPhone from the Mac Using the Bluetooth preferences, select the device you want to remove to highlight it, then click the minus sign. Then try reconnecting your iPhone to the Mac again.
After you pair the devices, a passkey is stored and authentication occurs automatically during future connections. The passcode is usually needed only once during the initial setup of the wireless device. Be sure to enter the passcode on the iPhone and Mac.
Use the Bluetooth Setup Assistant to set up your device to work with your computer by “pairing” the device with your computer:
If the Bluetooth status menu isn’t visible in your menu bar, use these steps to open the Bluetooth Setup Assistant:
After you pair a device with your computer, you should see the device listed in Bluetooth preferences.
I've done all that -reset settings, removed my iPhone from the list of devices, detected my iPhone and asked to pair it, a Passkey appears, my iPhone tells me that my computer wants to pair and asks me to confirm the Passkey displayed on my screen. I touch OK, the device briefly connects bunt then disconnects. My iPhone tells me that the pairing has failed. Where do I go from here?
Done all that, erased all content after backing up on iCloud, downloaded again and reset phone, trying to connect via Bluetooth gives me exactly the same results. The computer says "connected" then disconnects immediately. In top right- corner of screen "Network is unavailable". Phone says "Pairing was unsuccessful". What now?