Blocking Incoming Calls

The article will tell you how to block incoming calls to both landline and cell phone numbers.
To landline phones:

If you haven't already done so, sign up for caller ID (that being the feature that enables you to identify the number (and sometimes the name) of anyone who calls you.
Do not answer calls from phone numbers unfamiliar to you.
Join the national Do Not Call Registry. This prevents telemarketers from harassing you. There are two ways to sign up for the list:
a) Go to www.donotcall.gov/default.aspx and after you sign up, don't forget to check your email and validate the confirmation they'll send to you

b) Call (888) 382-1222 then follow the step-by-step instructions

Anyone with a US telephone number can place that number on the Do Not Call Registry. Just remember to allow a full month after receiving confirmation of registration before expecting unsolicited calls to cease. Once you register, your participation remains valid for a full 5 years.
Get yourself what's called an "inbound call blocker", a tool that enables you to screen your calls and choose which calls you will accept by providing permitted callers with a code that identifies them as an approved caller. There are many types of products that perform this service, including the following:

• http://www.commandcommunications.com/index.php
• http://www.privacycorps.com/products/
Your local phone company may also provide custom call screening services. Just check on prices before you sign up, as they may charge you per call, and the service could end up costing quite a bit. A cheaper alternative that the local phone company may offer is the option to rent a hardware device monthly to perform the same service.
Set up your phone with a "call screen" that will reject any calls coming from specific phone numbers. Blocked calls will be rerouted to an automated message that informs the caller that you've blocked their number. Again, be sure to check on the prices and pricing structure for this service before signing up for it.
Use a reverse cell phone directory to do a cell phone number lookup and find out who is calling you against your will, then take action to prevent them from calling you again.
To cell phones:

Use Caller ID to determine which calls to answer and which to leave unanswered. If a telemarketer leaves you a message on your voicemail, save it as evidence for potential future action.

If you answer a call from a telemarketer, let them know that they're dialing a cell phone number and inform them that making telemarketing calls to cell phones is illegal unless you, the owner, requests or consents to receiving such calls.

Sign up for the Do Not Call Registry as listed above.

Beyond this, there are as yet no official methods for blocking unwanted calls to your cell phone. However, if someone is harassing you, you can contact the wireless provider and request assistance with your problem from their security department.

How to Block Unwanted Incoming Cell Phone Calls...

References:

Roving Bug in Cell Phones Used By FBI to Eavesdrop on Syndicate.

wireless tracking.

Reverse Cell Phone Number Lookup