I hoard my cell number like a psychotic squirrel with a single nut in winter. When one icky person or telemarketer breaks through, it feels like they are penetrating my innermost sanctum and I'll do anything to keep them out. With the declining use of land lines these days, it's harder and harder to put a buffer between you and unwanted calls. Here's what I've learned for iPhones running iOS8.
The first level is to ban specific people and numbers from calling your phone in the future. The numbers have to be in your contact list first, so it's not the answer to all unwanted calls. But if you have an ex who won’t leave you alone, or any other tedious acquaintance, this will be helpful for you.
To add a number to the blocked call list, pull up their contact and scroll all the way down until you have the option to block that caller. Confirm in the next step.
You can also do this from Recent calls. Tap the "i" information icon to the right of the number, then scroll all the way down for the option to block the caller. Then confirm.
You can manage your blocked calls in Settings > Phone > Blocked. If you change your mind about blocking someone, this is where you go to reverse the directive.
The next step is dealing with the folks you don't know. This is what you need to do to thwart telemarketers, political calls, and fundraising efforts.
If they called you recently, but didn't identify themselves, go into your recents and find the unknown number. Then tap the "i" information icon to the right, scroll all the way down to block the caller. Note that their next call will go directly to voicemail, so you won't escape contact entirely. The good part about that is if it's actually an important call (say, from a medical provider) you'll still get the message. They won't be able to send you any text messages. They'll think they can, but they actually can't.
Even better news: iOS9 is rumored to have a new feature that suggests who the unknown caller might be. There aren't many details at this point, but it's a possibility.
Blocking calls, of course, won't work for people calling you for the very first time, or from a different number every time. (It's diabolical to be sure, but one particularly bad spammer calls me weekly using this technique). For that you have to seek out third-party apps like TrueCaller or TrapCall.
A more extreme method is to put your phone in the "Do Not Disturb" setting, and select all the people from your contact that you will allow to disturb you. This makes me nervous though, as I can see some people calling me for the first time that I actually want to hear from.
Your actual cell phone company might have added features, so check with them as well.