Here is an article and several Youtube videos describing an attack on the UK MI6 organization (their equivalent of the CIA). The article uses the term "phone bombing", where the attacker uses automation to flood the target with calls, thereby preventing legitimate callers from getting in. This is just another term for Telephony Denial of Service (TDoS).
The video's narrate the attacker calling in themselves and taunting the MI6. If this weren't such as serious type of attack, it would be humorous.
The PSTN is not what it used to be, closed and somewhat safe. It has so much VoIP/SIP in it, that it is starting to look like the Internet in terms of threat level. We will see more and more harassing calls, TDoS/phone bombing, voice SPAM, voice phishing (vishing), etc.