Attention:
Once phone call comes in, primary phone and secondary phones will ring together, just pick up any of them to start talking
You can use either primary phone or secondary phones to make a phone call, but its unavailable to make it at the same time
During the period of talking, no matter other phones are on-hook or off-hook, it will not affect the status of communication
During the period of talking, if you want another person to answer the phone call, just let him or her pick up the other phones