Generally many phone users think that there is an obvious inbuilt feature in iphone for blocking unwanted calls.
But unfortunately there is no such feature available in iphones. This function is controlled by the service providers. These providers charger a high fees of doing such function. If you still want to do that then you can do it. Although the process of doing this is not that easy, you can know from the article How to Block Calls on an Apple iPhone.
There is process by which you can block a call known as jail breaking. Now you need an application such as true caller or call blocker. But generally for iPhones iblacklist is used it costs around 12.9$ and provides you the ability to block calls and messages from some specific numbers.
After downloading iblacklist it will appear in your phone. Then open it and there is an option to add numbers to the blacklist, you can also add numbers to this via recent call list or you can manually add a number to it. After doing this set the call icon to ‘on’ and if you want to block messages the set message icon as ‘on’.
Lastly you need to configure your blacklist for the moment when calls come. There are options like blocking before your phone acknowledges the call, sending back the engaged tone, sending the caller directly to the voicemail, or automatically hanging up the call.
It is recommended that you must not try to jailbreak your iphone generally because Apple has made iphone secure by locking it. Jailbreaking can challenge its security. So if you to stop nuisance of the culprits then just change your number or make your phone on silent mode.