Blocking annoying phone numbers

[Gambas]

I have blocked several phone numbers from calling my residence, but some get around the block and keep calling.  How is this possible?

Second related question.  Do the blockec numbers expire after a certain period of time.

I am usind the fios app on my Ipad to block these numbers.

[tns2]

Its possible because the callerid system is archaic and was designed when there was no thought that someone other the phone companies would be adding such info to the call.   Regrettably its easy to fake out.