DIAGNOSIS The soundbar's built-in microphones, which it uses for your voice during calls, are likely being blocked or are picking up too much room noise and echo, canceling out your voice.
FIX This is about placement and environment. 1) Make sure you're speaking from your main listening position, roughly where you'd sit to watch TV. The microphones are optimized for that spot. 2) Before the call, press the 'Voice' button on the remote once or twice. This boosts the voice frequencies and can help clarity. You'll see 'Voice' on the display. 3) Reduce background noise. Turn off any fans, air purifiers, or the TV itself during the call. The mics are sensitive.
IF THAT DIDN'T WORK The soundbar might be in a 'surround' or 'music' sound field mode that widens the soundstage but scatters the microphone input. Press the 'Sound Field' button on the remote until you see 'Standard' or 'Auto Sound' on the display, then try the call again. This uses a more direct processing mode.
HEADS UP The microphone quality on a soundbar will never match a dedicated headset or phone mic. It's designed for convenience. For important calls, using your phone directly is still the best bet. Also, if you have the rear speakers active, temporarily turning them off during a call can reduce the audio loop the mics have to process.