We have a hacker breaking in through our voicemail, accessing an outside line and calling internationally. Has anyone else had this problem?

just last nite actually on a TVS50, they were not using external notification so i turned that off, also you can enable toll restriction system wide too.

Yep, happens all the time.
The "sniper" as he calls himself likes to target
Panasonic systems. He's been around for years.
Probably has message notification calling Manila
in the Philippines.
All the standard precautionary measures apply.
Password protect ALL mailboxes.
Delete the ones not being used.
If external message notification is not being used, disable it, if it is being used, restrict it to local only by assigning a restricted COS to the voice mail ports.
Good Luck, Gem

The "sniper" has gotten me a few times and the real trick is to put a password of at least 8 digits on mailbox 999. He needs to get in to this mailbox to create a new mailbox (somewhere up in the higher numbers)with the call forward to outside line so he can sell calls to the Philippines with just the cost of a local call. I have caught people talking on these calls. This is a for profit operation don't kid your self.