Calls from unknown numbers are more common than they were a few years ago. Telemarketing companies, creditors and business professionals all employ number blocking services for outbound calls. The problem with this is that it's harder to track someone who's harassing you, or call back telemarketers to tell them to remove you from their call list.
Talk with your phone company or service provider and find out what number blocking services they offer. Find out if the service is only for the local calling area or if it works with out of area calls, too.
Attach an inbound call blocker to your phone, which affects everyone who calls your phone. If the caller knows the code, which they will if you've given it to them, then they can ring through to your phone. If they don't know the code, the call is blocked.
Block specific numbers and anonymous numbers using Call Screen. This service activates by pushing 60 on your phone, once the telephone company has put this service on your account. When you get an unknown number you push 60, and a call from that unknown number can no longer ring through.
Have the phone company give you Anonymous Call Rejection, or ACR. This blocks all incoming calls that say unknown number, out of area or private. The caller must unblock their phone number before the call rings through.
Read through your cell phone manual for services offered on your cell phone. Not all cell phone providers offer an easy way to block calls, because they lose money. But some phones, like the Smartphone, allow users to block unknown numbers and anonymous calls.