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.