Sony DR-BTN200 Help Manual page 66

Note
If you received a call using your smartphone or mobile phone, some
smartphones or mobile phones may have handset use priority. In the case of the
HFP connection, talk on the headset by pressing the
for about 2 seconds, or using your smartphone or mobile phone. For details,
refer to the manual supplied with your smartphone or mobile phone.
To adjust the volume
Press the VOL +/– button.
Tips
You cannot adjust the volume when not talking.
Volume for a call and listening to music can be adjusted independently. Even if
you change the volume while playing back the music, the volume of a call does
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button on the headset