I would suggest you the following things ->Initially gather all the requirements and check with the client if you have any queries through email. ->Document every thing in MOM whenever you have client call and share with everyone who has attended the call(dev team,client,business,QA) ->Prepare a Test Plan strategy document, test cases and share it to client and request him for his sign off. ->Once, you are all set start with smoke testing then check the major functionalities in that release and then could proceed further. ->You could automate the regression test cases as you are going to execute them for every release(I would suggest to use Selenium if its a desktop application then UFT). Kindly, let me know if you have any queries.

Yes, that's a usual situation. Try to gather more info about new functionality function specification from product owner and developers