If you need to call someone who you don't want to get your cell phone number, you can block your caller ID. Depending on the carrier, your number will show up as Unknown or Restricted on the other end. You can block caller ID on a per call basis, or have your carrier permanently block caller ID on all calls.
Dial *67 on your cell phone's dial pad.
Dial the number you want to call.
Press the "SEND" or green phone button.
Your cell phone carrier can also block your caller ID permanently on all calls. Contact the customer care department about enabling this feature.
Some people have a feature on their line called anonymous call block. This feature blocks calls from unknown callers. If you call someone with this feature set up, you will receive a message stating you must unblock your caller ID.