Is the PSTN more secure?

I was just thinking, that perhaps one of the most advantageous features of the PSTN is it’s security :-). It is not possible to do a DOS attack (or other malicious activities) on a POTS line, unlike a IPPhone. (Or are all these issues resolved.)

For example is it possible for me to disrupt an existing conversation, but flooding an existing IP phone with requests? Ow! :-(. Or is this and others trivial scenarios?