Here's Ford's recommended procedure specific to iPhone.

Here’s how to pair your iPhone with SYNC with MyFord Touch:

• Start your vehicle and power on your phone.
  
  
• Enable your phone’s Bluetooth® feature:48
  Press Settings > Bluetooth
  Press the Bluetooth slider to turn on Bluetooth.
  
  
• Meanwhile, on your vehicle’s touchscreen, SYNC will have listed any devices that are already paired.
  Press Add a Device.
  
  
• Go back to your phone’s Bluetooth settings, and scan for devices.
  Press SYNC.
  
  
• Both your phone and SYNC should now display a six-digit PIN.
  Verify that the two numbers match.
  Press Pair on your phone, and Yes on your SYNC touchscreen to confirm.
  

Outside of this, one of the common recommendations with the XR is to turn off Bluetooth, restart the phone, and turn it back on.