In the workplace, as in life, I feel like the rules of communication are pretty universally understood. If you are unavailable to pick up a ringing phone, call the person back when you get the chance. Try to respond to texts and emails as quickly as possible, but don't kill yourself to respond in mere seconds or minutes to non-urgent communication. These rules apply to everybody, from the highest CEO to the lowest intern. If we don't communicate with each other, nothing gets done. Similarly, very little is accomplished by chastizing someone for not responding to you immediately when they're busy. Communication is a two-way street, and you can't expect one party to be constantly available if the other party couldn't care less about doing so as well.
A bank worker recently got into a tiff with their boss over this very conflict. They didn't answer their phone because they were training a new employee, but their boss thought that was an unacceptable excuse.