Zéro mobile network

[BTea]

Same here.

EE coverage map shows fully saturated 4G service for the area but the reality is a virtually unusable signal for months now.

-110dBm or worse most of the time.

Vodafone and O2 coverage is solid.

EE / BT repeatedly advise “use WiFi calling”.

[bristolian]

There will always be localised variations with all networks.

If your phone is having issues across wider areas, I would suspect a device issue.

[BTea]

bristolian,

You may notice I said the Vodafone and O2 are fine - on the same device!

And another handset shows exactly the same poor coverage.

[bristolian]

Some phones can still hold calls reliably below -115dBm, but it does sound like you've used the "check coverage" link rather than "check status" option

[cje85]

-110dBm isn't bad and should still be perfectly usable. 4G/LTE can work right down to -130dBm (sometimes even lower...)