No, you're on the right lines, but it was even more dastardly clever than that.
When a mobile's turned off it sends a 'goodbye' signal to the last mast it communicated with. So by finding out which mast received that signal, the police learned what area the phone was last used in.

Spookily, a small 'hot spot' including Huntley's back garden was the only place on the girls' last known route which was covered by the mast in question.

This impressive evidence helped persuade Huntley to confess.
(Juan Quar is clinging to her front teeth on, Fri 24 Oct 2008, 9:05, Reply)