Question: What is a key requirement for E.164 numbers assigned to Mobile Stations (MSs) in a Public Land Mobile Network (PLMN), to facilitate seamless calling from ISDN/PSTN subscribers?
Option 1: The E.164 numbers must be exclusively reserved for international roaming.
Option 2: The E.164 numbers should comply with the home country's E.164 numbering plan, enabling standard ISDN/PSTN charging.
Option 3: Assignment of E.164 numbers is not necessary if an MS has an X.121 number.
Option 4: Each PLMN must develop a unified global numbering plan for all its subscribers.
Explanation: This option is correct because complying with the home country's E.164 numbering plan allows for standard ISDN/PSTN charging to be applied for calls to MSs, facilitating seamless integration and billing for international and national calls, as implied by the need for any ISDN/PSTN subscriber to be able to call any MS in a PLMN.