The alarm is not The standard way of disabling the silent mode, the responsible of this task is the costcontrol app. 
The alarm is a security step, to prevent the silent Mode continues activated when it is not disabled by the costcontrol app. 
Moreover, in the worst scenario, if the new alarm fails, the following time that the costcontrol app try to enable the same type of service,  the system will try again adding the alarm.
 For these reasons, we continue enabling the silentMode although the method to add a alarm fails.