Unfortunately, this is the time of year when people fall victims to scammers pretending to be CRA and demanding money or you will be arrested or something similar. Fortunately, in Canada there is a long process before anyone is arrested for unpaid tax bills.
The first thing you should do if you are being called by CRA and are unsure is to hang up and call CRA back on their official phone numbers listed here at the personal enquiries page. Any real agent with CRA will not have an issue with you telling them you would like to call them back.
CRA has also put out a specific page to address these scams, it is well worth the time to visit and read the page called Don’t Get Scammed.
Finally if you are still unsure, call a relative or someone you trust to help you deal with any kind of phone call you think might be a scam.