There are unlimited number of threads on similar queries at LCI that you can search in 'Search' option.
The mployees that are united and have formed unions/IC/Guild/Works Committee etc, have affliiated with trade unions can handle such situations much better.
There are unions of BPO/KPO/IT/ITeS employees now and have affilliated with trade unions like CITU/INTUC/AITUC/BMS etc..........................Search in Google.
The first thing first: Explain the situation in writing to next employer under proper acknowledgment and conclude that until or unless the servic certificate/relieving letter,acknowledgment and a ceeptance of resignation is supplied to you ,you can't provide these and company should absord you on the strength of notice of resignation,proof of delviery only and must not terminate you if you are unable to supply these, and should buy out notice period/pay and pay it at once upon joining.
You may ask the current employer to compute earned wages, bonus,incentives,reimbursments,leave encashment,notice pay etc in FnF statement and supply it to you. If notice pay in computed in FnF statement and is remibursed by next employer yu may be double taxed.
You may also relate evertyhing with clauses of ( name of your state) Shops and Establishments Act,Model Standing Orders,Certified Standng orders and the notice period of 90 days may not neccessarily be applicable to you. It may not be more than 30 days.You may demand copy of all above documents from company in writing under proepr acknowledgment and now write by letter thru Redg. psot and /personal email id and download copy of all communications sent from official email id.