Voice mail calling

Sometimes I think voice mail has a mind of its own. I'll pick up my messages, then hang up the phone - and the next thing I know, my phone is ringing, and it's my voice mail account. Today that happened after I left a message for my sister on her voice mail. This time the ringing phone was Suze's voice mail. What!?!

This is a real curiousity (to me). Does anyone know why Verizon's voice mail seems to insist on not being finished when I hang up the phone? Is is an idiosyncracy of Verizon's system, or do others share the same behavior?