If you've been inundated by nonsense phone calls, you know that it's wise to limit who you give your phone number to. I discontinued my land line specifically because of cost and the annoying calls (mostly from a political party who refused to remove my phone number from their database).
But are you required to give out your phone number? Even though, since 2009, it's illegal for companies to use autodialers, they're not restricted from overseas numbers and many online forms require a phone number without letting you know who is going to be using it.
Also, a company you have previously done business with (business relationship) is entitled to call you, even if your number is listed on the Do Not Call Registry.
See information about stopping unwanted calls on the Federal Trade Commission site here
Stores may also ask for your phone number when checking out. I always respond with "I don't give out my phone number."
So what is the solution?
Most of the time if they insist and won't be put off with "I don't give out my phone number," I use a fake number (my old phone number currently gives the message that the phone has been disconnected if someone calls).
For cell phones, you can put the number in your reject call list or find an app to block calls.
Other alternatives are to set up an additional phone number on your phone. These usually come with an additional monthly fee though - around $4. Or you can sign up for a voice mail only service.