There are several different methods of blocking a phone number from one's cell phone. These methods depend on the cell phone's model, operating system and service provider. Certain models of cell phones may not even have the capability to block unwanted numbers.
To block a number already in one's iPhone Contacts, navigate to Settings, select the Blocked option, and select Add New... To block a recent caller on an iPhone, navigate to Recents in your call history, tap the circled I icon next to any given number, and scroll down to the Block option.
For Android phones, the process of blocking phone numbers is more complicated; many Android cell phones do not allow one to block phone numbers without the use of an additional application. For Android phones that allow this process, navigate to the phone icon,and select Call Settings. If applicable, select the My Device option, followed by the Blocking Mode option. After this, select Disable Incoming Calls. Navigate back to Blocking Mode, and then select Allowed Contacts to choose which numbers will be allowed to reach the cell phone.
Another option for Android devices is to select the Settings option from within the phone application itself. Select Call, then Call Rejection, followed by Auto Reject List. After this, select Create, and add the phone number or contact to block. If this option does not work for your Android device, pull up the device call log. Tap the three-dot menu icon, and then select Add to Reject List.
If none of these options appear to work on the device, contact the phone's service provider for more options, or download an application to get the job done. One such Android application designed to block unwanted phone numbers is called Call Control.