Accused of using company phone whereas a personal iPad was used
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.
2,Tap FaceTime.
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.