I'm asking on behalf of my friend regarding his HTC ONE M7.
The phone is running fine on WIFI but, there's an issue with the network service. It takes a while for the phone to pick up the network operator and when it is connected to the network, it shows full bar with 3g signal. The issue is that, my friend cannot make a call or browse the internet using data network. Even when he tried to check his credit balance, it would sometimes show the balance and sometimes it will display and error with "Invalid MMI code" or something. Same thing goes to making a call, sometimes he can call, sometimes he can't.
I tried to make a call from my phone to his phone and sometimes it's ringing and again, sometimes his phone can't be connected.
He flashed new ROM but the issue is still there.
Any of you guys have any idea on how to fix it?