On 870 only defines which keys to show in call screen. It must contain 6 keys, all are shown, those that do not work in a given scenario appear disabled. The 3rd key may be replaced by 'activate call completion' button under certain circumstances.
A space separated list of F_-keys (e.g.: F_CONFERENCE).
F_REC F_FAVORITES F_TRANSFER F_VKEY F_HOLD F_EDIT_ADR_BOOK