Bug 914599 - Improve Call Settings when FDN is enabled

When the Fixed Dialing Numbers options is enabled, many features are
disabled, and USSD/MMI codes fall into this set. This will thus make the
many call settings parameters getting into error state. The present
commit aims at: avoiding undefined state of the call settings, and
improving the feedback we send to user by clearly stating that call
forwaring options are unavaiable due to FDN enabled.