What can one tell from a person's voice (on the phone)?

I was calling an ex-girlfriend (wild hippy chick in her day) some years ago and the line was always busy. I began to think she was making money on the side by answering a 900 number using her home landline somehow. When I "confronted" her with my suspicions, she was taken aback and denied ever considering such a gross endeavor. Heck, I would have given her credit for taking the initiative for finding an easy source of income.