I’m in Cuba and my phone keeps ringing - Thought I wasn’t supposed to be able to receive calls here

With fizz you can be charge for something you dont have in your plan and they say down this page that you can make ( or receive )  call outside Canada and USA. So if it's working, it's suppose because you have access  to it, than try to answer !

You want to use your cellphone while abroad?

1. To use your cellphone during your trip abroad, you need to buy a Travel Add-On or again add funds to your wallet to pay as you go.

Can’t decide if you need a Travel Add-On or should pay as you go? It depends on how long you will be away and how much you will be using your phone while roaming. If you will be travelling for a long time, choose a Travel Add-On (it’s more affordable, and will be valid until the end of your next payment cycle). If you’re going on a short trip, paying as you go will be more practical.

2. Make sure to activate the roaming option of your phone, or else it'll block your usage. 

Important : You will not be able to make calls while roaming outside your Canada or the United States.
Roaming calls (incoming or outgoing) will be available at a later time. We will update this FAQ when we have a firm date.
In the meantime, you will be able to use data, and send/receive texts when you travel outside your coverage zone. To make and receive calls, you can use messaging apps such as WhatsApp or Messenger.

Fling said: "With fizz you can be charge for something you dont have in your plan..."

He probably meant to say you cannot be charged for something you don't ahve in your plan.

Enjoy.