Hi iafuj,
We appreciate you reaching out to us. We understand that the person on the other line is unable to hear you if you answer your phone call on your Apple Watch.
First, let's test if the microphone on your Apple Watch is working as expected.
Please record some voice memos, then check if the recording is clear.
If the recording is clear, it is likely that your device's microphone is working as expected.
Please check for a software update and make sure that you are running the latest version of iOS and watchOS.
While it isn't inherently assured that an update will resolve the issue, updating to the latest software version for your devices can help isolate potential software issues.
Kindly secure a backup and update.
How to back up your iPhone, iPad, and iPod touch
Update your iPhone, iPad, or iPod touch
Update your Apple Watch
If the issue continues after the update, please unpair your devices, then pair them back again.
Performing these steps may help correct any feature that is not working as expected.
Unpair and erase your Apple Watch
Set up your Apple Watch
If you are unable to get a clear recording, please make sure that nothing is blocking the microphone on your Apple Watch. For example, a protective case
Please make sure that the microphone is also clean and free of any dirt or debris.
It may also help if you unpair any Bluetooth headphones from your Apple Watch if you have any.
If none of the situations above applies, a technician may have top take a look at your device's microphone.
You can use this link to explore your service options: Get Support
Take care!