Apple has acknowledged that some units from its latest flagship iPhone line have faults in which they cannot emit sound during calls. The OEM has accordingly set up a new service program by which users affected by these issues can send them to office centers to have certain components replaced. It applies to the 12 and 12 Pro variants only.
https://www.notebookcheck.net/Apple-announces-a-new-program-to-replace-faulty-call-speakers-in-the-iPhone-12-or-12-Pro.557200.0.html