A lot of people—those in tech, and creatives even moreso—prefer, and default to asynchronous, written communication.
Here’s the thing though: async, written communication assumes that the person on the other end is privileged with single tasking, or able to focus shift between doing something, and reading messages and replying; i.e. that they are sitting in front of a computer, or staring at their phone.
If you are messaging me on Slack, you are making an assumption that I am either not doing anything, or I'm able to stop what I’m doing and answer. But what if I’m driving? What if I am doing something which makes typing impossible: on a bicycle, carrying things, or simply having my hands or attention occupied?
My advice is to consider the job and general responsibilities of the person you are trying to reach, and choose the communication channel to be in line with that—even if that means actually dialling a phone number. They will appreciate it.