There has been much talk lately about VoIP security.
The FCC has a page to warn consumers and businesses alike about Voicemail hacks. (The FCC calls it Voice Mail Fraud but it's really about hacking the voicemail system to enable toll fraud.) This actually happened to me a few weeks ago. I never go into my vmail system because I get all my vmail as email. Remind your users. Or change it yourselves.
This article about the 7 Types of VoIP Fraud is pretty interesting.
The Internet Telephony Service Providers' Association has developed some guidelines for secure deployment of IP-PBX. [PDF].
I know that you think you know it all and that you have the master genius in your NOC. However, everyone can use a reminder. Furthermore, most companies do not have documented processes and procedures or things like a checklist (which would include demonstrating voicemail and making them change the password on Day 2). This results in practices that have holes. Holes get exploited. Exploits cost you customers and money.
Another reason to develop procedure documents: it helps with scale. As you grow, you will have the processes written by the personnel that were most familiar with the procedure. McDonald's isn't successful because they have great fries. McDonalds is successful because they have systems and procedures in place that any $7 per hour person can perform. They don't need a chef or a cook. They just need someone who can follow directions off a laminated card. That is the secret to the success of McD's.