Feature codes, sometimes named star codes are special phone-numbers a user can call to activate or deactivate some feature in a pbx. This can be done with an actual call, but we also have multiple predefined key-types that call these numbers in the background (silent) without the user noticing an actual call:
Our auerswald pbx'es also allow to establish a sip-subsbscription of type dialog to most of the feature-codes so that the phone gets feedback whether a feature is currently switched on or not to properly reflect the current state via the led of the key that is configured.
Every pbx-system uses it's own feature-codes. If you use the d-series with a non-auerswald pbx and want to use feature codes, then you should look at the existing templates and change their feature codes to match your pbx.