My boss is going to sack me tomorrow, because he says I used my company phone whilst driving the company van. I definitely didn't use my phone. Sometimes I might audio FaceTime my girlfriend on my iPad (which has a different SIM card and is not a company iPad.). And she may phone me back but even though it rings on my phone it also rings on my iPad. How does this happen?
Facetime will ring on any device you have registered your email address for the Facetime.app on that device.
There is an explanation here on imore.com which goes into some details on how it works and how to register, enable and disable an email address on a device.
How to enable or disable phone numbers and email address for FaceTime
If you don't want your iPad or iPod touch to receive FaceTime calls sent to your iPhone phone number, or don't want your work iPhone or Mac to receive FaceTime calls sent to your personal email address, you can easily disable -- or enable -- them on a per-device, and per-address/number basis.
1, Launch the Settings app from the Home screen of your iPhone, iPad, or iPod touch.
3,Tap on the checkmark to the left of any number or address you want to turn off (prevent from calling FaceTime on that device).
To enable or re-enable numbers or addresses, simply reverse the process and tap the empty space to the left to number or address to add a checkmark.
If you always carry both devices then there is no need to set up personal accounts on company equipment.
If the boss's entire case is based on a mobile call during a driving period (rather weak, you could have been parked / stopped etc.) then the whole situation is probably for another reason and this is just an excuse that will pass the labour board.