Samsung HF4000 User Manual page 16

When the hands-free car kit is connected with two phones, this function is
not available.
This function is available only on the Hands-Free profile. For more
information, see your phone's user manual.
Answering a second call
Press the Talk button to end the first call and answer a second call of the same phone
when you hear call waiting tones.
Press and hold the Talk button to place the first call on hold and answer a second
call of the same phone when you hear call waiting tones.
To switch between the current call and a held call, press and hold the Talk button.
When the hands-free car kit is connected with two phones, this function is
not available.
This function is available only on the Hands-Free profile. For more
information, see your phone's user manual.
Answering a new call of the second phone
Press the Talk button to end the first call and answer a second call of the other phone
when you hear call waiting tones.
Press and hold the Talk button to place the first call on hold and answer a second call
of the other phone when you hear call waiting tones.
To switch between the current call and the held call, press and hold the Talk button.