If your iPhone 12 or iPhone 12 Pro emits no sound from the receiver when making or receiving calls, you’re in luck. (Relatively speaking, anyway.) Apple has opened a repair program for what it says is a “very small percentage” of those devices experiencing such an issue and will fix affected iPhones for free.
Affected units were manufactured between October 2020 and April 2021, but there’s no need to look up a specific serial number to check eligibility; just contact Apple Support if you’re experiencing the problem. You can also make an appointment at an Apple Retail Store if there’s an open one nearby or contact an Apple Authorized Service Provider. The repair program doesn’t apply to the iPhone 12 mini and the iPhone 12 Pro Max, which presumably don’t include whatever substandard component is failing.
The repair program is valid for up to 2 years after purchase, and the program doesn’t otherwise extend your iPhone’s warranty. Also, if your iPhone has any other damage, you may have to pay to repair that before Apple will fix the sound issue.