As long as you do not formally accept the job by clicking the 'Accept' button when the client sends you a contract, it is fine to engage in conversation. After the discussion that you need to keep at Upwork - including voice or video calls - if you like the job and its terms, inform the client that you are prepared to begin working as soon as his/her profile has been verified. In short, only accept a contract when the client's payment method is verified. Sometimes clients are trying to be fast and hence working concurrently, trying to identify suitable freelancers as verification of their payment method is being processed.