Apple's iOS offers a native blocking feature for users to block unwanted phone calls on their iPhones. To add a contact or phone number to the blocked list, you need to tap the "i" icon next to the person or phone number you want to block to open the contact information screen. Once there scroll to the bottom and tap the Block this Caller button.
Some Android phones, such as the Samsung Galaxy S5, allow users to block specific callers through the phone's settings. You can do this by tapping the phone icon on the home screen of your phone and then select your contact list. Once there tap the three-dot menu to pull up the Settings menu where you can create an Auto Reject List and add the name or phone number of the caller you want to block to the list.
For Android phones that don't offer the native blocking feature, you need to download and install call-blocking apps. Some call-blocking apps for Android are Mr. Number, Call Control and Extreme Call Blocker.
Another option to block unwanted calls is to contact your carrier. As of 2015, Verizon's free call-blocking service allows customers to block up to five phone numbers for 90 days. AT&T charges $5 per month, as of 2015, for its call blocking service, which allow customers to block up to 30 phone numbers.