Mysterious phone calls

Sometimes the mic on a phone breaks/doesn't function all that well, quite often mobile pones cos all sorts happens to them - when I worked help desk years ago there was a particular Nokia that seemed to have this problem when it was knocked... Sometimes it was an older person, or someone not particularly savvy or confident on the phone that had the problem - they only made a very small amount of calls and didn't realise that there phone was bust for ages... Older people tend to think something is up with the 'line'.

Could be that - or maybe something like the mute button on a land line is bust... Just a thought.