With most mobile phones, you must contact your service provider and have them block phone numbers you don't want calling for you. If you use an iPhone 3G, you may also employ this method. Thanks to the iPhone 3G's capability of handling applications, however, you can actually block and unblock phone numbers yourself --- so long as you "unlock" your phone first, which entails downloading a program onto it that allows it to be used on any mobile network.
Unlock your iPhone 3G, if it is not already unlocked. Point your browser to "blackra1n.com" and download the "blackra1n" application that corresponds to your operating system. Launch "blackra1n," then connect your iPhone 3G to your computer using your USB cable, making sure to close iTunes if it opens. Click the "Make It Rain" button on your screen, then wait a few moments for the application to transfer its data onto your iPhone, which will result in your iPhone restarting itself. Tap the "blackra1n" icon on your iPhone 3G and then the "sn0w" icon within "blackra1n" to unlock your phone to work with any network, keeping in mind that your phone must be connected to the Internet --- via your mobile network or any Wifi network --- to complete the unlocking procedure.
Install "Cydia" on your iPhone 3G. "Cydia" is an alternative to Apple's "App Store" that allows you access to applications specially intended for unlocked phones. Tap the "blackra1n" icon in your unlocked phone, then "Cydia," then the blue "Install" button which will display in the top-right corner of the "blackra1n" interface, keeping in mind that your phone must once again have access to the Internet. Wait a few moments for "Cydia" to be installed.
Download "iBlacklist" from the "Cydia" store. Tap the "Cydia" icon and tap "Cydia Store: Products." Type "iBlacklist" into the field and tap "Go." Tap the "iBlacklist" icon to download and install it onto your iPhone 3G. Wait a few moments for installation to complete. Your screen will refresh.
Block the number or numbers of your choice. Tap the "iBlacklist" icon and then "Active List." Tap "Add from Contacts," "Add from Recent Calls" or "Add from Recent SMS" and select a phone number to block. Tap the red "Phone" icon next to a number to block it. You will no longer receive calls from this number on your iPhone 3G.