My understanding is that the phone will definitely access the cellular network
It will try but for the most part won't succeed because there is little or no signal at altitude.
This is why a lot of "modern consumer" GPSs work so poorly when in the air. Their GPS is optimised for low power consumption and is compromised so that adequate performance is obtained with the assistance of the GSM data. The Iphone/Ipad is one example.
I would make sure the phone is ON and has obtained a GPS fix before you take off, because it may fail to get it afterwards.