After multiple lines are configured on the 7960 and 7961 IP phones, when the receiver is picked up in an attempt to avoid an inbound call on a secondary extension and make another call from the main line, the IP phone connects to the inbound call. The IP phone does not require the selection of the line in order to answer a call that rings on a secondary line.
In order to resolve this issue, change the values in Always Use Prime Line. The Always Use Prime Line system parameter affects the line settings on the IP phones. This parameter specifies whether the primary line on an IP phone is selected, if available, when the phone goes off-hook.
If this parameter is set to True, when a phone goes off-hook, the primary line is chosen and becomes the active line. Even if a call rings on the second line, only the first line is active. It does not answer the call on the second line. In this case, the second line must be chosen in order to answer the call.
If this parameter is set to False, the IP phone automatically chooses an available line as the active line.
The default value is set to False. In order to change it, complete these steps:
Open the Cisco CallManager administration page.
Choose Service > Service Parameter.
Choose Cisco CallManager from the second drop-down menu.
Scroll down to the Clusterwide parameters and look for the Always Use Prime Line parameter.