They called 4 times within a few minutes of each other and said they were from CRA and said that it was serious and they need to speak to me right away. Called # back and it kept hanging up.

Its a scam. The number is probably phones. CRA will not call you, and if you owe money they will always contact you by letter....specifically snail mail. Give them no info, send no money.

Called to say that I am in trouble with Canada Revenue.  When I call back, it keeps hanging up.

Message left from (613) 927-9665 - said from Rev Can and I was to call back before they take action.  I don't owe Rev Can  and they have not tried to get a hold of me.