I now have a Moto G5, with the Android operating system (v7, Nougat), which I am using with GiffGaff (a mobile network that in fact runs on the O2 network). The app and the phone work beautifully.
I have rung friends, rung family, and rung organisations. And it works! Folk who know me are taken quite by surprise, because they do not expect to hear my voice on a mobile.
It is quite amazing to be able to use a phone ‘almost’ normally. Liberation!