If memory serves me correctly Poole has come up before.
The My Network App suggests a busy network, in other words there is a lack of capacity. In that situation any fault will be keenly felt.
Use WiFi Calling at home if you can
https://www.o2.co.uk/connectivity/wifi-and-4g-calling
I would try PAYG SIM cards from the other networks to see which is best in your location, then if possible move.
Guide: How can I sort out my network issues?
Guide: Is the network down for me or everyone?
This is not O2 and we are all customers here similar to yourself and cannot answer account type queries.
