Claims to be Officer Gary Foster from CRA and that I owed back taxes.  The initial call is a recording.

got a voice message from Gary Foster  from CRA. Slight accent and poor quality recording, not a live person. Claims to be from Ottawa. Phone number is from a VOIP service in Markham.

What is the call back number? I want to waste their time

Received call from "Gary Foster" of Canada Revenue and told to call back at 6138007535 and that it was time sensitive. Very poor recorded message. Obviously a fishing expedition.

I received a call from that number yesterday and Gary Foster left me a message. At the end he got aggressive and told me not to try to disregard the call. I had just read on the Revenue Canada website that there are  scammer trying to get your Id, Bank Machine info and so on, so I disregarded it, but today I called Revenue Canada. They got all the information that I have.
Unfortunately I received another call today from "Inspector Gary Foster" from a different number 613-817-9481 with the same
message. These people change telephone numbers more often than their underwear.