Ghost calls

All of the 7 phones in our office have been recieving calls from “asterisk” twice an hour, 20 till and 10 after the hour. The caller id just says “asterisk”. I’ve looked through support forums and searched the internet but haven’t been able to find a solution for this. Any help or tips for solving this problem would be greatly appreciated.

What does the asterisk CLI say when the calls are being made?
What types of phones do you have?
Do you have analog interfaces? What kind?
Does it only happen when there are unread voicemail messages?
Turn on SIP debugging to see the messages that are being passed around.
If it is due to MWI, try removing mailbox=xxx from the peers in sip.conf.

Which card are you using?