You can share any contact information you want privately with a client as long as PAYMENT stays on Upwork. You can communicate through email, phone, WebEx, Zoom, whatever, as long as all payments occur through Upwork.
Many of us have to create logins to use for client tasks. I actually have emails with the client's domain name that they've assigned me to deal with different logins. As long as all payment happens through Upwork, it's fine.