In 2007, I spoke at an international cellular conference in Italy. The big buzzword was ARPU - Average Revenue Per User. The seminars all had titles like, "Maximizing ARPU In a Digital Age." And yes, several attendees (cell executives) admitted to me, point-blank, that the voicemail instructions exist primarily to make you use up airtime, thereby maximizing ARPU.
It's time to demand that cellcos offer the option of omitting the 15 second "voicemail instructions" from cell phone messages. In the meantime, you can take control of your own phone by following these instructions.