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.

In your situation I would be honest with them and explain what you wrote in your question. I don't think it'd be pushy to ask why Slack didn't work for them; maybe they wanted to do something niche that Slack couldn't support. That doesn't necessarily mean they should rule it out altogether.

In my experience people generally tend to be reluctant to start using new everyday tools, and the more you try and force them to use something, the more resistant they become. After all, there's nothing "wrong" with their current approach; it works and it's what they're used to, even if it does have problems. It can be improved, sure, but first you have to get them past their current mentality that it'll be too much effort and not worth switching. Stuff like communications tools/approaches typically aren't considered important when there's real work to be done, but having good communication is key to producing good work in a team.

You could try showing them interesting features of these apps, e.g. MS Teams has various extensions like polls. Not to mention they usually have voice and video calling built-in.

Starting small and building them up to using it is likely the best approach. If you keep asking them to use it, they will probably hear you saying: "I think we should completely switch from our current approach to this new one," and they're not going to cooperate. So maybe start by having video calls on an app like that, maybe even between only one or two people. Over time, if they use it consistently and enjoy it, find it easier etc., then they will naturally switch to it. But don't expect them to switch instantaneously; I had a similar situation in my group; and it took months before people used MS Teams as their main method of communications. What we have now is MS Teams for 90% of comms, and email for anything special that doesn't really fit into Teams.

tl;dr start small and build them up to it. Don't keep bashing them over the head with it. Ask questions and find out what's not working, and explain how there may be solutions or improvements elsewhere.