A：This issue is generally related to the following aspects:
1. Adapter design limitations: Many USB-C to
3.5mm headphone adapters are designed only for audio output and do not transmit microphone signals. That is, they lack internal circuitry to handle microphone input, causing the phone to default to using the built-in microphone during calls.
2. Standard compatibility issues: Wired headphones primarily use CTIA and OMTP wiring standards, which differ in the arrangement of the microphone and ground wires. If the adapter and the headphones or phone use different standards, the phone may not recognize the external microphone and will select the built-in microphone.
3. System recognition and software settings: Some phones prioritize the built-in microphone in call mode to ensure voice quality, especially when no clear external microphone signal is detected. Hardware and software compatibility can also cause the phone to use its own microphone even when an external microphone is available. In summary, when using this type of adapter, if the adapter itself does not support microphone signal transmission or has standard incompatibility issues, the phone will default to using the built-in microphone for calls.