I've created a TAC SR regarding this. They contacted the development team who say it's an error in the documentation, not in the phone. The display is apparently too limited to display the line text label.

I've got the same problem and according to the user manual it should display the line text label if the phone is idle:http://www.cisco.com/en/US/docs/voice_ip_comm/cuipph/6921_6941_6961/8_5/english/user_guide/phone.html#wp1009166Line details and othe...