Why does only one headset work when the headset is plugged into the phone?

Why does only one headset work when the headset is plugged into the phone?

When the headset is plugged into the phone, only one headset works, please refer to the following two situations:
1. Poor contact
You can plug it back in and try turning the headphone plug. Please try another phone, if the problem persists, please contact the after-sales service.
2. Mono phone does not support dual-channel headphones
Some phones have only one sound channel. If the headset is dual-channel, only one headset will have sound when the headset is plugged into the phone.