Cognizant — regarding release of my offer letter

I interviewed on 4 th april 2019 and got selected for pv, location pune and it is being told that i will get my offer letter on 22 nd of the april 2019 but till date i did not even get any update from the company not only this i had mailed several times but there was no reply, also i called on office number but guess what no one even recieved the call. My brother also given got selected for developer post and his experience was even worse than mine, and one of my best friend also faced this situation, he has been told to came down to office so that they can provide them the offer letter after got selected and waiting for almost 5 months, he travelled good distance and went there 1 st day they make them wait for around 6 hours and next also for same duration and on 3 rd day he got to hear that there is no vancany now... I mean is it make sence
One more thing i would like to mention that before giving interview for pune location, i was given interview in cts kolkata for pv that time i was working with other campany which i wanted to change due to my personnel reason so went to cognizant i was done with every round and got selected, feeling very excited and happy after that i got a call from hr and she was demanding resignation letter and experience letter for make it possible i quickly resign to that company and provided my all required documents to the hr, now i am left with only hope that i will get my offer letter she told that i will take around a week i thought its ok after 1 week i mailed her but no reply than i called her did'nt anwered i called twice but no answer i kept calling her next day and this time she told the she is not sure about my joining and she hang up on me.
Even after all this i went to give interview in pune just thought of i was only problem with kolkata location but i was wrong.
This is not fair how can a hr demand resignation letter of last company when he or she not sure of joining of that candidate
I lost my last job and did'nt even got the job.
This time also they are repeating the same story i would like to request to take some strict action against this.
If there is no vancany than why they provide walkins.
I beg you pls do somthing about this.