Apple Watch series 1
When someone calls or I call someone I can hear the caller but they can’t hear me volume is up all the way
Apple Watch
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Best reply
Are you sure the microphone is on?
Tap 
to mute your end of the call (if you’re listening on a conference call, for example).
Can you use Siri? See the 'If Siri doesn't respond or asks you to try again' section in:
Use Siri on your Apple Watch - Apple Support
See if the microphone or speaker on your Apple Watch is blocked. For example, if your device has a protective case, remove it.
If callers still can't hear you, contact Apple Support.