SteveR21 Loc: Oklahoma

I am with those who recommend using the iPhone flash to trigger your off camera flash. Most speedlights will allow optical triggering - the flash will fire when it 'sees' another flash of light.
Make sure the receptor for the speedlight is turned so that it will 'see' the iPhone flash. The S1 setting will fire with any flash (including a preflash for focusing or red-eye reduction). The S2 setting will ignore the preflash and fire with the main flash.

Haydon

The Godox line has a smartphone app for their line of lights but you'll have to check for compatibility and have the latest firmware installed.

DirtFarmer Loc: Escaped from the NYC area, back to MA

I don't use the phone for many photos but I have tried to use the built-in flash with it. As I recall, setting it to flash produces about a 0.5-1.0 second light from the phone, followed by a brief flash. I assume that means the phone takes a bit of time to establish exposure (or focus). Don't know how a long light pulse would be interpreted by a remote flash and whether adding an off-camera flash would cause overexposure since it wasn't included in the initial light pulse.

Time for some experimenting.