You may see your phone show orange lights on all of the line buttons. While this is happening, all of your calls will be forwarded to your voicemail.
What this means is that you’ve placed your phone in a state which forwards all calls to voicemail. (For more information on states, see our Call Forwarding with States article).
To get rid of the orange lights and return your phone to normal operation, press |State|.
Then, use the arrow keys to choose Available and press |OK|.