Any system that is that widely anchored in society is a valuable target.
- Before everything was IP-based you could occupy a large number of lines and making it impossible for more calls to go through (i.e. 911). It's called TDoS and could be achieved through phreaking.
- You can spoof your caller ID to make your scam more convincing.
- You know how when you call your voicemail from your phone you're not asked for your PIN? The voicemail system only checks your caller ID to know it's you and skip the PIN. So, again by spoofing your caller ID and calling the voicemail number you can access listen to anyone's voicemail. This doesn't work on all providers, many have now reluctantly fixed the problem.
I recently came closer than I'd like to falling for a scam (read: I picked up a call and conversed with the caller for 30 seconds before realizing it's a bot), simply because the notification for automatic call screening[0] displayed a summary of ongoing conversation, which happened to look very much like caller ID - it said "Name Surname, Department of Security, ${my wife's bank}". But it wasn't caller ID, just a bad interaction between the way the scam bot introduced itself, the summary feature of the call screening feature, and the UI design of the notification...
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[0] - A thing Samsung has on recent phones, where LLM picks up a "potential spam/scam" call in the background and engages with it, while producing a transcript and recording you can review as it happens.