Apple doesn't give the option to block numbers, and third-party apps can't do the job because - unlike with Android phones - Apple doesn't allow third-parties to access the call-making capabilities of your phone. It's actually a good thing, as it means no third party can make money by sending spam from your device. But it means that any app that claims to able to block a specific number is probably not fit for purpose: as witnessed by the one-star ratings such apps garner in the app store.
Indeed, even the telco that provides service for your phone is unable to help. Here's a response from GiffGaff too a forum posting asking how to block a number on an iPhone: "There isn't a way [to block calls] because apple have locked the iPhone down, which makes it so secure, but does mean that apps can access the phones inner workings. GiffGaff don't offer this function. You could change your number if you wish."
So, to stop nuisance callers you can either change your number, or jailbreak your iPhone. Below we show you how to do the latter.
How to block a number on an iPhone1.
First, jailbreak that iPhone. We don't actively recommend this, and we do recommend that you take time to weigh up the pros and cons.
2. Now you need an app to do the job. There are several, including 'Call Blocker' and 'Call Bliss'. You won't find all of these apps on the Apple App Store - for some of them you'll have to use the independent Cydia app store. The blocking app we are going to use is called iBlacklist. It costs $12.99 (about £8.50) and offers the ability to block calls and SMS messages from specific numbers, as well as all unknown numbers. Open up Cydia, then find, pay for and install iBlacklist. Restart your phone.
3. iBlacklist should now appear as one of your apps. Open it up, and click 'Blacklists'. Here's you'll find all your current blacklisted numbers - at this stage there won't be any. To add one hit 'Add new Blacklist', then add a number. You can do this via your recent call list - which is where you are most likely to find the culprit. You can also add in a contact, or manually put in a number. Once you've added a number, set the 'Call' icon to 'On'. To block text messages do the same thing with the 'Message' icon.
4. Now you need to configure what happens when a call comes in from that blacklisted number. Hit 'Action to be performed' and choose an option. Options include blocking before your phone even acknowledges the call, sending back the engaged tone, sending the caller directly to voicemail, or automatically hanging up the call. Finally, hit the blue button to save your selections and that number is blocked.
5. To block other numbers or even groups of contacts repeat steps 3 and 4.