There have been some user reports on the Internet for some time now that there are issues with some iPhones that affect the operation of the speakers on the phones. Users report that the speakers do not emit any sound during phone calls. Apple identified this problem and in order to fix it offered users free repair of phones with this problem.

Online reports indicate that this problem occurs in some iPhone 12 and iPhone 12 Pro units. At the moment, Apple informs that the problem may concern phones manufactured between October 2020 and April 2021. All devices can be repaired completely free of charge at authorized Apple services. Importantly, only the 12 and 12 Pro models can be repaired within proposed program. At the moment, Apple has informed that free repairs will not apply to the 12 mini and 12 Pro Max models.

source