From what you've posted, it sounds like you're referring to an irregular illness where a teacher needs to phone in to say they are ill at the time, as opposed to an ongoing situation where the teacher wouldn't need to phone in every day (and would most likely have a medical certificate stating they're unfit for work). The two are different circumstances & would elicit different expectations. Are you referring to a longer term illness, or just the day-to-day kind? If you're physically/mentally not capable of supplying cover lessons, then wouldn't your medical certificate state that? Is there anything about long/short term illnesses in your contract/working conditions? You could certainly contact your union to find out - they could probably give more accurate advice on your specific case.