Below is from support.apple.com. And here is the link again.
About Hearing Aid Compatibility (HAC) requirements for iPhone
Activate Hearing Aid Compatibility
To activate Hearing Aid Compatibility on an iPhone with iOS 13 or later, go to Settings > Accessibility > Hearing Devices. On these iPhone models, Hearing Aid Compatibility modifies the phone’s acoustic settings to improve compatibility with hearing aids set in “T” or telecoil mode.
If you have an iPhone 6 or earlier, you can activate Hearing Aid Mode. To activate Hearing Aid Mode, go to Settings > General > Accessibility. Hearing Aid Mode reduces the transmission power of the cellular radio in the GSM 1900 MHz band, which may result in decreased 2G cellular coverage.