It depends how you define dangerous, I suppose.

- 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.