Communication must stay on upwork until the contract is set up, after that, you can communicate however you prefer. Upwork has a zoom feature in the message center itself if you guys want to make calls. Thats what I use.
Most of my clients work with emails most of the time and communicating on email makes their workflow much easier. Also, they check their whatsapp much more so they can answer me faster.
I always keep the payments and contracts on Upwork but sometimes the client needs some communication before moving forward with the contract and they have a hard time doing it on Upwork.
So Is it okay have a quick chat with them on whatsapp or email before setting up the contract? I always make sure to keep everything on Upwork.
Again, as listed in the TOS, until a contract has been initiated, all communication must stay on the platform. After you start the contract, you can communicate wherever you want. There is really no reason a client that is hiring you on Upwork cant message or call you on Upwork prior to starting a contract.
By hindsight, I believe it's best to keep all communication with clients on Upwork since all they are doing is to protect us from scam. I would have been a victim of scam right from my early days as a freelancer, but for the intervention of Upwork. I give kudos to the Upwork platform for their good deeds.