Mobile coverage investigation with my new smartphone.

[Mike Watterson]

CW is correct - emergency calls can be taken by any network, not just your own, so yes, you could be out of coverage, but still able to make an emergency call if you're in the coverage area of another network.

As long as SOME network has coverage.

Some operators additionally have roaming agreements.

The efficiency of mobile was been crippled by Government greed to chop up and "sell" spectrum. It is not like other things, because that reduces the capacity. The only sane way to do mobile is have a single wholesale operator for all bands/channels and if desired have multiple retail sellers. A single RAN per country.

Some traffic analysis will show the present system wastes capacity. It's absolutely stupid for such a small fixed resource.

[Mike Watterson]

Here in Ireland, I either have coverage, or none.

[CambridgeWorks]

I only recall seeing either normal access or emergency calls only.
Never seen zero access.
Rob

[stevetech99]

I use an android app called Signal Strength to measure signal strength and also obtain a lot of info on surrounding cells. 
To get data on wider signal strength Ofcom publish a lot of data and also their methodology here .

Cell capacity is a different problem and sadly is not something you can do from your mobile phone.