Incorrect 1099-B with cost basis showing zero

I had some RSUs/ and ESPP from previous employer. Recently the company got acquired and the RSU/ESPP stocks are settled in Cash & Stock settlement. That is 50% is settled in cash 50% is settled as a converted stock from new employer. Even though this transaction was all correct, the financial where I have the stock plan account didn't include my actual cost basis in the 1099-B. I am suppose to have loss on the amount settled in cash but as the financial institution is showing my cost zero in the 1099B, I am suppose to pay tax now for this amount. What is my options ? I am not very hopeful to get any help from financial institution as they are saying it is a problem from my older employer and My older employer is not responding to my query in this regard. What is my options now ? Shall I manually calculate everything as per the actual cost basis ? Or I should go with the amounts mentioned in 1099B. Kindly give some guidance on this