Samsung SGH-I777ZKAATT - Manual (Page 54)

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Call Waiting
The Call Waiting feature allows you to answer an incoming call
while you have a call in progress, if this service is supported by
the network, and you must first activate the Call Waiting feature:
ᮣ Tap ➔ Settings ➔ Call ➔ Additional settings ➔
Call waiting.
You are notified of an incoming call by a call waiting tone.