Sony MDR-1RBT - Manual (Page 70)

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 button on the headset
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.
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