Your argument seems to be that he should be instantly available on his telephone but not on zoom or slack? Seems like you're just shuffling the problem around.

I think that's pretty reasonable. Most people at the end of the day are available whenever via a phone call. But if you use it for something trivial, they will be pissed. Seems like a decent system to me.

In this age of mobile phones, a phone call is the gold standard for realtime business communication. It's pretty reasonable to expect someone to answer their phone if their direct supervisor calls during work hours. Text messages are more fuzzy with the implied expectation of response time. Online messages (IMO anyway) shouldn't be considered a realtime channel.

It was entirely my fault to be over reliant on notifications.

In some places they even push the expectation of being accessible immediately at any time ("being on call"), and it’s not paid hours. Crazy times we live in.

who doesn't pay "on call" though?

I've never been paid extra for 'on call', always been salaried and had it added as an expectation after the fact.

I'll admit I've quit over it.

I am an underpaid (for my area, experience, title, coworkers, etc) software developer who has occasional on call shifts and once a year holiday shifts with no extra pay or benefits

Management is never paid for being on call in my experience. It's just expected that you can be called any time for any reason.