My preference is still Slack, even in this case. Send a message prefixed "URGENT" and I will write down where I stopped in the next 30 seconds and then get to you nearly right away.

I can skim the first few words of a Slack message without losing context and if those first few words demand an immediate response, I can "save" what I am doing to my notes.

I had a co-worker who did this in a past job, even though he sat behind me. It was quite helpful.