Question: How are unused nibbles coded in an Emergency Call Code with less than 6 digits, as per the USIM application's EFECC specifications?
Option 1: Set to '0'
Option 2: Set to 'F'
Option 3: Set to 'FF'
Option 4: Vary based on the preceding digit
Explanation: In the context of emergency call codes within the USIM application, coding efficiency and clarity are crucial. When an emergency call code is shorter than the maximum 6 digits, unused nibbles must be consistently filled to maintain format integrity. According to the specifications, these unused nibbles are set to 'F', ensuring easy identification of actual code lengths without confusion or potential for misinterpretation by the network or user device. This approach aligns with efficient binary coding practices in telecommunications.