For Android phones:
STEP 1: Tap the phone icon/widget to go the dialing screen.
STEP 2: Tap the "menu" button which is on the lower left side of your phone, beside the Home button.
STEP 3: Go to > Settings > Call > Call settings > Additional settings > Call waiting
Step 4: Check the box to activate.
STEP 1: Go to Settings > Phone > Call Waiting
STEP 2: Slide the toggle in the right to activate