For PC there are various apps to control amount of kilobytes transfered, like Netmeter, DU, Bandwidth Manager, etc.
But for smartphones, or PC- Mobile, I ave not found any.
I know that the provider is overcharging communications, but to prove it it's not so easy, I have made comparisons using the same apps, Messenger in the PC, but it's not the same.
The app would need to measure and list links and respective usage.
Thanks for help.