Cognizant — I'm complaining about offer letter

Address: Electronic city phase 2, Bangalore, Karnataka, 560100

I have been selected for cts held on 16th nov 2019 in bangamane techpark bengaluru for react js position and hr promised me i will get offer letter within a week. I got a confirmation mail that i was selected and you will get a call from recruiter at the earliest. But till now i haven't received any call or mail regarding the status and i tried to reach out hr through mails or calls and i got no response from them and now they are calling me for the same position. I was in confusion whether to attend the interview for same position or still i need to wait for offer letter and when can i expect offer letter