Your VM has probably been hacked. Did you leave it to a default code? Contact your carrier, have it reset and be sure to enter a different password.
Frauds are starting to hack VMs, especially if they've left incriminatory messages and suspect you may be on to them. My company has encountered this on a couple of occasions.
Be safe. Be aware.

There's a voicemail but it says my password is now invalid. Never have had this problem before in the 4 years I've had this phone or #