Hi,
I just purchased the Galaxy Watch5 Pro LTE version and I was also scratching my head over this issue.
I haven't tested LTE calls because I don't have the option yet with my carrier.
But as for the Bluetooth version, I just figured why it doesn't work and actually this all makes perfect sense.
Plain answer: "You just can't passthrough audio from one Bluetooth device to another."
As a matter of fact when the watch is connected via Bluetooth to your phone, the watch actually behaves like a handsfree Bluetooth handsfree kit, hence the fact you hear the sound through the speaker of the watch.
For what we want to achieve to be possible, the audio from the call would have to be streamed via Bluetooth from the phone to the watch, THEN from the watch to the buds.
I guess that's just not possible with the current Bluetooth technology.
Just like the fact you can't have two devices connected at the same time to one Bluetooth audio device: for instance, you can't stream music from your phone and hope to be able to hear exercise audio guide from the watch at the same time.
It's sad but that's the way it is: unless you have a fully autonomous LTE version, you'll have to choose between pairing the buds to your phone or your watch.
It's just too bad the documentation isn't specific enough about this restriction between the LTE and non-LTE version:
