From the Phone app, tap Keypad.
Enter the number you'd like to call.
Tap the Call icon.
You can see your call's signal strength in the notifications bar at the top. The more bars that are filled in, the stronger your signal is.
To add aditional callers tap the More icon.
Tap Add People.
Select the desired contact you'd like to add to the call.
After calling the new contact tap Merge to combine the calls.
To hang up, tap the End icon.
You've completed the steps!
