I never used to sing, I was completely off key all the time I tried. Then I joined a covers band about ten years ago and was asked to sing back up, when I told them that I couldnt sing they didnt care! There were two vocalists in there already, so I hid behind their voices.
Fast forward ten years when I joined a new band, I was trying to get the guys to do a cover of Creedence Clearwaters 'Fortunate Son' and the singer couldt quite nail it. So I had a go at it myself to demonstrate it and really surprised myself!
The band where so impressed they said 'Wow! you sang that really well, why dont you sing it?'
So all those years singing week in week out, I trained my voice without me even knowing it!!