What region do you work in? The laws are pretty different depending on where you are.
In most places, they can't make you answer outside working hours - that counts as overtime and has to be paid or compensated somehow. The law usually protects your right to rest. That said, if your contract specifically says you're working "on-call" or in a standby mode, that can be legal, but then it has to be spelled out and paid separately.
The smartest move is to check your local labor laws first or get a lawyer's advice, then approach management with facts. If your contract allows it, you can get your agreement or disagreement in writing - that's insurance. But if the contract doesn't include those obligations, then legally you just turn off your messenger on weekends and that's it.