I think the word you want is "reception".

It refers to how well the mobile phone (handset) connects to the mobile network.

If you stand in the middle of the forest in Siberia = "no reception".

You can also say "mobile coverage", referring to the area where signals ARE available.

Sketchy reception=an area where you may or may not get coverage. It varies.
Poor mobile coverage=the mobile phone gets little or no connection.

Remember, use "mobile" for British, Australian and NZ usage. North America uses the word "cellular".