Diane is getting odd phone calls from her VOiP system at work. And the caller ID has only a few digits. Leo says it could be the provider upstream. But with VOIP, she could also have a security issue. Leo recommends finding a consulting IT guy who can lock down her system.
According to the chatroom, there is a hacker tool called SIP Vicious, which exploits the SIP protocol that VOiP phones rely on. It scans the internet looking for SIP phones and seeks to break into the phone network to place long distance calls. RingCentral has a good tech note here - https://community.ringcentral.com/ringcentral/topics/annoying-ip-calls-from-1000-1001-120-etc
(Disclaimer: RingCentral is a sponsor).