Personally i think you should be careful giving out customers addresses to other clients with out asking them first, it's a bit of an invasion of their privacy.
I certainly wouldnt want a tradesman telling half the road what work I've just had done.
It would be ok to say one of your neighbours, but i dont think i would put down their number unless i had thier express permission to do so, and i dont think many of my customers would give it .
Regards
steve