Samsung ATT_SGH_I727_CELOX_UM_KJ1_WC_102711_F7 - Manual (Page 55)

Call Functions 50
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.
A checkmark will appear in the box when activated. You
are notified of an incoming call by a call waiting tone.