LG G5400 User Manual page 23

You can make a second call while you are already in a call. To do
this, enter the second number and press S [Send]. When you get
second call the first call is automatically put on hold. You can swap
between calls by selecting <[Options] then
.
You can connect a call on hold to the currently active call to form a
multiparty call by selecting <[Options] then
.
To put a multiparty call on hold, select <[Options] then
.
To make a multiparty call on hold active, select <[Options] then
.
To join an active call to the held multiparty call, select <[Options]
then .
To scroll though the numbers of the callers who make up a
multiparty call on the handset screen, press D and U keys.
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