I am writing this on behalf of my mother who is struggling to no end with her current service. She works as a Care assistant and needs her phone to work 24Hours a day which currently is not even close to reality.
When making outbound calls she is unable to even hear the regular tone and most calls end with Server Unreachable message, when she is able to make a call she is generally unable to hear the other caller or she gets a very intermitted call quality. The same can be said for text messages which she is lucky to receive at any time, normally coming hours or even days after they where sent originally.
I have tried changing directly to iD APN settings and disabled WiFi calling, her current SIM has also been in another phone which was exactly the same.