Should a person file and pay taxes on 1099r pension from death where box 2a shows unknown when it is believed the funds were taxed before placed into pension?

Husband had money pretaxed before placing into pension. Husband past away and wife is receiving pension however it states in box 2a unknown. Wife was told she needs to file and pay taxes on pension she has received of $15,000 but she knows this money has been taxed before placed into pension account. She has made many attempts to call the company  handling the pension account but has not been successful. It has been years now that husband has past and she has been told she must file and pay taxes on the pension received all these years but she knows she should not have to. She is 79 and is discouraged and upset that company handling the pension has placed her in this type of situation being unfair.