Options during a call, Putting a call on hold, To make a call while you have a call in progress – Samsung SGH-X427 User Manual
Page 51: Press the key to dial the second call, Make the second call in the normal way

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Options During a Call
Your phone provides a number of control functions that you
can use during a call.
Putting a Call on Hold
You can place the current call on hold whenever you want.
You can make another call while you have a call in progress
if your network supports this service.
To place a call on hold, simply press the Hold soft key. You
can reactivate the call whenever you want, by pressing the
Unhold soft key.
To make a call while you have a call in progress:
1. Enter the phone number that you wish to dial or look it
up in Phonebook.
2. Press the
key to dial the second call.
The first call is automatically put on hold.
Or
1. Put the current call on hold by pressing the Hold soft
key.
2. Make the second call in the normal way.
To switch between the two calls, simply press the Swap
soft key.
The current call is placed on hold and the call on hold is
reactivated so that you can continue conversing with the
other party.