The only thing free while roaming is receiving texts. Anything else has high pay per use charges or you can pay 13 dollars a day for easy roam to use your phone as normal, and be allowed to call Canadian and Spanish numbers. It's capped at 150 dollars per billing cycle. Depending on how long you'll be in Spain, it may be better to get a local prepaid Sim card.

I don’t want to call, just being able to receive calls to that number. 

I already have a Spanish number, but since this is also a business number I want to be able to attend business calls to my Canadian number while in Spain. Only for receiving, not sending. 

You will be charged roaming for ANY call while roaming. This applies whether you are placing OR receiving the call.

The ONLY thing that is free while roaming is receiving text messages.

If you don't pick up the call you can receive (rings) the call request. You can note the phone number and call back with your Spain phone number.