In addition to the information and warnings given below, as a general rule:
Keep communications on Upwork until you have a contract, after which you may communicate however you and the client like. Keep all payment transactions and as much discussion of payment as possible via Upwork channels.
And the general exception:
Once contact has been established, you may use any means of communication you like (e.g., some clients prefer working out hiring details via email). Don't divulge personal information, and don't end up giving away your time or doing free consulting: "In general, yes [no]. I'd be happy to discuss details once we have a contract..."
+1 for Michael's response.
Once you have a contract signed and a job in progress if you and the client prefer to communicate via email or Skype, it is wise to send copies of same to your workroom. That way it can not be disputed should you stumble upon a less than honorable buyer.