Check your cell phone address book or contact list if you are calling from your cell phone. Sometimes people can save individual numbers to where some people can see who is calling and some can't. You want to enable the phone number for the person you are dialing.
Look to see if 67 is placed before a number when dialing. If you are calling a previously saved number--and wanted it to be blocked--you likely saved 67 before the number. Simply click "edit" and remove it from the number. When you dial again, it will show your number as unblocked. This will enable your name to pop up on your friends' cell phones if they have it saved in your contact list.
Call your telephone service provider. Oftentimes, when you sign up for phone service, the company asks if you want your phone number to be listed in the phone book. If you opt for it not to be--usually for an additional fee--they will automatically show your phone number as private when it's used. When it's listed publicly, you might get more telemarketing calls, but your number will no longer be blocked.
Check the settings of your cell phone. While this varies both by cell phone and individual service provider, there are usually options under the "Settings" menu. You can choose to block your number entirely or not at all. Under initial set-up, you may have chosen to have your number blocked. If your friends are now complaining that your number is appeared as "Unknown" on the caller ID, explore the "Settings" again.
Enter *82 prior to the phone number you are calling if you only want to unblock your number on a call-by-call basis.