You are basically just making your number available to the world.
Depending on what other details are there with it (name, address, interests, sex, children's names, pets names) you open yourself up to various forms of abuse and potential actions from people that might like to (say) ring you and tell you your children have been injured (for example).
Don't ask me why - I don't understand it - but some people really seem to find it amusing.
Never had a seemingly nonsense call like that? I can assure you it can have a devastating affect.