How to motivate team to use a communication tool?

In my team we do not use any professional communication tool. We are working totally remotely because of the COVID-19 situation. All the communication goes through email or Whatsapp personal accounts.

Email is really inconvenient, threads get really long, so difficult to follow or quote anything. In addition, the inbox look like get filled quick and spammed.

Whatsapp was not made for team work, is a tool for personal communication and me personally I feel uncomfortable to having to share my private account to talk also with my co-workers.

They told me in the past they were using Slack but "it did not work" for them and they had to fallback to use whatsapp.

I do not want to look too pushy, specially if they say they already tried Slack. I believe using another similar tool such as Microsoft Teams would feel the same as Slack for them. Probably no gain. I honestly do not know how they cannot realize communicating over email and whatsapp is really not efficient for a team work.

Any ideas on how I can motivate them to use any tool? And glad to know what tools you think would be suitable, i.e. what tool is so easy to adopt that they won't drop out? It would be something in the direction of Slack / Microsoft Teams.

Since the team has already tried to use Slack with little success, the team is unlikely to respond to persuasive arguments. However, you can start to dictate how the team communicates with you.

Set expectations on your communication habits:

1. Set your Whatsapp status to "Find me on Slack @Username"
2. Add your Slack username to your email signature with something along the lines of, "For an immediate response, ping me on Slack @Username"

Change your communication habits:

1. When someone pings you on Whatsapp or email, start taking just a little bit longer to respond. Not drastically longer as to jeopardize your job or sabotage projects, but start with an extra 10 minute delay and add a few minutes each week
2. When someone pings you on Slack respond immediately

With this approach, you're letting your co-workers know that you are on Slack, that you might prefer Slack, and by responding much faster via Slack that they may want to prefer that channel when communicating with you.