Why is my Iphone letting through calls from blocked contacts?

I got a contact that I have blocked on my Iphone 5. It contains several numbers (70-ish. It is all from telemarketers) But i sometimes still got calls from some of thoose numbers. In my screen it even pops upp the name i gave that number. But the numver i sblocked according to all rules. I check that everytime it happens.

So

- is there a limit on how many numbers you can put on a contact and still get it to block? Is there other settings i Need to do?

Are there ways for telemarketers to overwrite a blocked number in my phone?

This is a problem I have had for more than a year and thorug many updates

Many questions...

(sorry for bad english- not my native language)