As it happens, I've never used Slack.
I've been coerced to use all kinds of ad hoc collaboration methods, but I use two of them: email and SMS.
Email is is a storage medium, and SMS is a real-time medium. If you are trying to make your problem my problem, use email. If you use SMS, you will get blocked, your problem is not my problem in real time.
If you could fix the problem, you already have access to the solution. If you don't have access to the solution, you're an intermediary, and your problems are not my problems.