Another good reason to retain the guest's phone number is that people happen to be forgetful and they do leave their stuff behind once they check out.
I specify in my House Rules that I may be able to mail them their items agaist the cost of mailing + handling fee. This has helped my guests understand that leaving something behind and having it returned to them comes at a reasonable expense to them. Amazingly, since I introduced this rule, no one has left anything behind. Great! A little incentive to be careful has helped us all!
Please, upvote my post if you find it useful!
Thanks,
Danny