[wheels]

I read the grandparent differently than you and nightpool.

I read the first two paragraphs as general policy, and then the following lines as specific cases. To come to the interpretation you guys had, you have to assume that the fifth paragraph supersedes the first. It's possible that's how the author meant it, but that's not obvious. The first paragraph implies virtually always preferring calling, independent of which direction the help is flowing.

Addendum:

I don't love Slack, but my company is small, and the number of messages per day is in the 10s. Nevertheless, I have notifications disabled for all messengers, because I also find them interrupting. I only see messages when I'm switching windows anyway. I have the same on my phone: I only have visual notifications. I disable sounds and vibrate. When I'm either being particularly productive, or particularly struggling to focus, I close all communications apps.