The best protection is to not connect it at all.
My advice is get an ESP and use that as your messenger.
If that burns then “no harm” done.
As I see it you could either set up a voltage divider and a diode bridge to convert it to DC.
Or depending on position of the bells and general loudness in your house.
Add a microphone and “listen” for the bell.
Notice the section that product is under?
Solid state relays.
Optocoupler is a type of relay.
It’s pointless arguing with you, after all my 20+ years experience as an electrical engineer means nothing. A traditional magnetic relay cannot leak between the coil pins and the switching pins.
Mostly for scenarios like the OP mentioned. To galvanically isolate signal paths with different voltage or ground potentials to avoid damage, ground loops and similar. And that at very high frequencies. Relays are for low frequency switching higher power loads, not data signals. Using a relay is not a good choice for the OPs situation, but well. It’s inevitably going to fail at one point or another, especially the cheap ones.
Never say impossible. Look at the thing the OP found on AliExpress above. We’re talking about the cheapest of cheap, scrapping the bottom of the barrel of bad quality, $0.01 counterfeit relay here. Everything is possible with crap like that. There is no guarantee for appropriate isolation between coil and actuator/contact side. I’ve seen crap relays like that where the coil heated up so much that the internal isolation melted away. That said, with a good high quality relay, leaking between coil and contacts is very unlikely, unless the relay suffered heavy mechanical damage.
Oh and optocouplers are not a type of relay. They’re not a type of solid state relay either. An optocoupler is a type of galvanically isolated transistor. A solid state relay uses MOSFETs or Triacs. It may use an optocoupler as part of its circuit to provide better isolation, but the relay function itself is performed by the MOSFET/Thyristor/Triac part.