I have a Sony M4 Aqua Dual E2333. Bought it in 2015. I flashed it with the latest official Marshmallow firmware, using the official utility, over a year ago; the version is still latest. I have done afactory reset a few days ago, but the problem did not go away.
Sometimes I get a weird situation. The phone is showing decent coverage,a few bars, 4G. Mobile Internet is working. However, I can not make calls - I get "mobile network not available". And calls also do not get through to me. (I guess my network - Tesco Mobile Ireland - does not support voice over 4G).
When I reboot the phone, remaining in the same location,I get perfect 3G coverage as well as 4G and calls work fine. So this is not an actual coverage issue. And the problem is critical, because it means sometimes people can not get through to me.
Is this a hardware problem, do I have to replace the phone? (It is way out of warranty). Or is there a way to fix this?
Another problem with the same phone is that sometimes it loses Wi-Fi and will not find it again until I reboot the phone. This is not critical, I have enough mobile data. Mentioning it becaue it might help diagnose the issue.
