No, but it is extremely rude to do. I almost lost a friend because she did that to me.
No .. It is not illegal for the normal average person to 'share' someone's phone number .. However, dependant upon where you live .. If your are a business or an employer (for example) who has attained a phone number through means that fall under the scope of privacy laws, then, it may be illegal to share it.
I believe much depends on the realistic 'expectation' of privacy. In any event, it is not an 'arrestable' offence. You could potentially be charged for breach of privacy and usually a stiff monetary penalty is the consequence.
As Bikergirl Anonymous said, it may depend on how you came to know what it is.
As a courtesy, I don't give out phone numbers for individuals whose numbers I know without aksing them first if they mind.
no but its social law, you should know that phone number is still a part of private sector, same thing if you share what color your daddy's underwear is wearing now.
A lot of the answer depends on the purpose for giving it out. For purpose of a future criminal act and you do it knowing what will happen, then yes but if with no criminal intent, no. Now the intent or lack thereof here is almost impossible to prove in Court so many DAs stay away from prosecution here