Sudden drop in signal - S7 edge

beer enigma

Over the past couple of days I've had a pile of messages saying that I've missed calls yet the phone hasn't rung. People tell me that its going straight to voicemail.

I called Three and they checked some stuff and got me to do a network reset, but same issue. I've just noticed that I only have 1-2 bars in places where I'd normally have a great signal. this has happened in both Dublin and Cork, so can't be a transmitter issue.

Anything else I can try to get the signal back or is this likely to be a phone/sim issue ?

Phone and sim are 9 months old