Hi Wee,
Just to add a note to the great info from Alex (+5 "A")
Your reference to "dual-line" indicates to me that you are using CME,
if so, then this is working as expected. You will want to look into using
Octo-Lines
First introduced in Cisco Unified CME 4.3
Octo-Line
An octo-line directory number supports up to eight active calls, both incoming and outgoing, on a single phone button. Unlike a dual-line directory number, which is shared exclusively among phones (after a call is answered, that phone owns both channels of the dual-line directory number), an octo-line directory number can split its channels among other phones that share the directory number. All phones are allowed to initiate or receive calls on the idle channels of the shared octo-line directory number.
Because octo-line directory numbers do not require a different ephone-dn for each active call, one octo-line directory number can handle multiple calls. Multiple incoming calls to an octo-line directory number ring simultaneously. After a phone answers a call, the ringing stops on that phone and the call-waiting tone plays for the other incoming calls. When phones share an octo-line directory number, incoming calls ring on phones without active calls and these phones can answer any of the ringing calls. Phones with an active call hear the call-waiting tone.
After a phone answers an incoming call, the answering phone is in the connected state. Other phones that share the octo-line directory number are in the remote-in-use state.
After a connected call on an octo-line directory number is put on-hold, any phone that shares this directory number can pick up the held call. If a phone user is in the process of initiating a call transfer or creating a conference, the call is locked and other phones that share the octo-line directory number cannot steal the call.
From this good doc;
http://www.cisco.com/en/US/docs/voice_ip_comm/cucme/admin/configuration/guide/cmebasic.html#wp1105668
Cheers!
Rob
