This problem occurs when the user hits Cisco bug ID CSCsa63770.
The Always use Prime Line parameter allows the user to designate a line that will be active when the phone is off-hook. When the cluster-wide service parameter "Always use Prime Line" is set to true and the phone goes off-hook, the primary line becomes the active line. Even if a call rings on the user's second line, going off-hook makes only the first line active. The phone does not answer the call on the second line.
However, when this bug is present, the phone does not use the prime line when a second line is ringing. If the user picks up the handset, the phone answers the call on the second line.
To resolve the issue, install the appropriate CallManager version, or downgrade the IP phone firmware.
This bug is fixed in these Cisco CallManager versions: