Your Polycom VVX 310 has 6 lines. Each line can connect to 10 callers. Only one active line — the line that has an active call and audio associated with it — can be in progress. Other calls are either held or placed in an incoming or ringing state.
To place and answer your calls, you can use one of the following:
Press the headset button to use a connected headset.
Press the speakerphone button to use the speakerphone.
To change to the handset, pick the handset up.