It'll be some scammers from India or somewhere else using VOIP to fake legit phone numbers. CRA and RCMP won't do much.
Best way to stop them is to waste their time.
Answer the phone and say, 'omg, don't worry, I'll pay, just hold on a minute'. Then put them on hold. If you can take the time to pick up the phone and continue wasting their time they'll get pissed off and leave you alone. Just add in 'I'm just looking for my tax return' or 'I need to go find my SIN card' every once in a while to keep wasting their time. Just see how long you can keep them on the phone.
And its fun.