Sony SGPT21 - Manual (Page 47)

Notes on Using Voice Communications Apps
When you use voice communications apps such as those for Internet telephone or voice chat in the following
cases, both parties cannot talk simultaneously. Speak alternately.
A headphone without microphone function is connected to the headphone jack.
Nothing is connected to the headphone jack.
You can talk simultaneously when a headset is connected to the headphone jack.
When you attempt to use Internet telephone or voice chat with 3G, you may not be able to establish a
connection. Use those with Wi-Fi.