An on-call contract is a flexible employment contract in which an employee works on an on-call basis and not on the basis of a fixed number of hours. The employer calls the on-call worker only when he needs them and, in principle, only has to pay the actual hours worked (with a minimum of 3 hours). An employee working under an on-call contract does not automatically enjoy the same rights as someone with a permanent employment contract.
Incidentally, there is no legal definition of an on-call contract. An on-call contract is also called a zero-hour contract, minimum-terms contract, or MUP contract (an employment contract with a deferred duty of performance).