For example, on some android phones, the data connection shows as on when you are on a call, even tho apps cannot access the internet.
So I'm assuming the OS just built a workaround that turns data on only for calling, since if data is allowed to be completely off, people are gonna be complaining that their "Phone isn't working" as most carriers require VoLTE these days.
Same thing with location. Even if you turn location off, it is automatically turned on if you call emergency numbers.
When you make a phone call, the antenna's are used 100% for calling and using data is disabled (or at least, it used to be). Android might thus "pretend" that it is still connected.
Data is definitely usable when you make a VoLTE call. But if you turn data off before the call happens, the data icon on the top quick setting panel where you pull down on the notifications indicates that data is "on", and attempting to turn it "off" would show a message saying "Unable to turn off data while Advanced Calling is on" Advanced Calling which I assume means VoLTE. So basically its this state where its on, but only for the purposes of VoLTE. If you have data set to on before the call happens, you can look up info while, for example, you talk to a customer service representative.
Well the cellular network has voice over LTE or voice over NR, whether the data side is turned on or not. So unless it's an airplane mode with Wi-Fi turned on and you go out of range of the Wi-Fi, it should just switch it from the Wi-Fi over to the cellular network.
VoLTE doesnt count against your data plan, you dont even need a data plan to use VoLTE. In fact, most carriers require VoLTE these days so when you first insert the sim card, your carrier has sent a configuration profile instructing your phone to turn it on. You can turn the data to "off" but it still leave it on for VoLTE calls only.
I know the SIM gets some information when the phine is turned on. I have never had data service on phone before, and I gave up cell service entirely awhile ago, so I was wonding how the iPhone could stay connected for service that has never existed or been used on phone.