When you are in a call, you can hold your current call and place a new call. Only one active call (the call that has audio associated with it) can be in progress at any time.
STEP 1
During a call, tap back arrow to return to the Calls screen.
STEP 2
Tap on the Calls screen or tap to search and then dial out the contact. The active call is placed on hold and the new call is set up.
STEP 3
The active call is placed on hold and the new call is setup.