Part time or on call

Hi there, I work in a LTC facility in Ontario. As a disclaimer, I am planning on leaving as it was a gig to support me through school.

I’m a part time employee; my contact has a weekly hour commitment (24 hours). Problem is, I’m inconsistently scheduled for that minimum requirement.

Here’s the kicker which I’m looking for perspective on: every time an extra shift comes available, they keep track of how many I refuse. This includes when people call in and then I get a call last minute to cover. I received a warning that if I refuse any more shifts, I’ll be penalized.

Excuse me if I’m wrong, but does part time mean on call? Is it my fault that my official scheduling doesn’t have the minimum hours needed so therefore any shift that does come up, I have to take?