Which plan was you on? How many minutes did you have in your plan?. Was it unlimited? 
Also, I would double check the pdf bill (check the page 3 please) and see if it was a long distance calls or “accidental roaming” charges. 

@October2nd Your plan only includes 500 minutes per month. If your needs have changed you may want to upgrade to an unlimited minutes plan. 

Thank you guys for response! So, is Airtime charged for domestic calling as well? I thought it counts when I call outside of Canada.

@October2nd Yes, airtime is counted against just about every call whether domestic or not. That is why plans with unlimited minutes are popular. 

Calling outside of Canada is not covered by the 500 minutes or unlimited minutes. It just doesn't charge you extra beyond international call rates.