Remember, probabilities are not inherent facts of the universe, they are statements about how much you know. You don't have perfect knowledge of the universe, so when I ask, "Is your mum on the phone?" you don't have the guaranteed correct answer ready to go. You don't know with complete certainty.

But you do have some knowledge of the universe, gained through your earlier observations of seeing your mother on the phone occasionally. So rather than just saying "I have absolutely no idea in the slightest", you are able to say something more useful: "It's possible, but unlikely." Probabilities are simply a way to quantify and make precise our imperfect knowledge, so we can form more accurate expectations of the future, and they allow us to manage and update our beliefs in a more refined way through Bayes' Law.