The series of letters that appear in the top left of the display are used to indicate the status of various features.

idle display


B = Barred
A B is shown on your phone's display when the system administrator has set you to outgoing call barred status. You will only be able to make internal calls while this is applied.
D = Diverting (Forwarding) Calls
A D is shown on your phone's display when you have forward unconditional enabled.
G = Group Member (In Group)
A G is shown on your phone's display when have been configured as a member of a hunt group and your membership is enabled. While this is the case, you may receive calls targeted to the hunt group.
N = No Calls (Do Not Disturb)
An N is shown on your phone's display when you have do not disturb enabled.
O = Out of Service
An O is shown on your phone's display when any of the groups of which you are enabled as a member is set to night service mode. In that mode calls to that group are diverted to its fallback if set or otherwise to voicemail if available.
T = Twinned
A T is shown on a phone's display if it is internally twinned with your phone. Calls to you will alert on both phones and can be answered by you at either phone.

S = System Alarm
If you are configured as a system admin you will see the letter S to indicate the system has an alarm.