A number isn't as dangerous as putting something like your home address online, which is plain stupidity. As far as phone numbers, all someone can do is call with threats, or call and be annoying, etc etc. It just depends if you want to go through that. If you wind up with a stalker it can be highly annoying, where you actually have to end up buying a new simcard for your phone, or permanently leaving your phone on silent or off. It can get dangerous when you start answering stupid questions like: what do you look like, where do you live, can we meet etc...so in my opinion, putting any personal information on the internet is plain stupidity, and DOES have the potential to become dangerous.