hacking cingular voicemail

now before i start, i just wanna say that i nor anyone reading they should try to do anything illegal with this info.
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I was calling up a buddy this afternoon, an he didnt answer an it went to his voicemail,blah blah it got the the part where it says leave your message after the beep then it beep, well instead of saying something i started to wistle an at a certain "tone" the phone said message deleted.
...i thought for a second what just happen, an the phone proceded to restart an say leave a message again. so again i wistle at that certain tone to make sure it wasnt a fluke an again the phone restarted an said message deleted. i even spoke a couple lines then wistled to make sure it didnt do that automaticly.
so i hung up called back but this time before it beeped i wistled up different pitchs an this sime it said please enter pin number, hes not the smartest one so after some research i figured out the default is like 1111 or something, an with that it let me into his voicemail, were i listened to his messages an such then called his home number to tell him.

now.. i know back in the 70s when guys used "phreaking" to get though the switchboards an such. but i thought phones companies went digital or whatever an u couldnt do that stuff without pressing certain buttons an all?