Question: What is the required format for representing a global telephone number as a Public User Identity in IMS?
Option 1: SIP URI with "user=id" parameter and userinfo part following the Tel URI format
Option 2: Either a Tel URI or a SIP URI with "user=phone" parameter, following E.164 format as defined by ITU-T Recommendation E.164
Option 3: Diameter protocol with canonical form "diameter:username@domain"
Option 4: Local number with "phone-context" parameter in a Tel URI
Explanation: This is correct because, when the user identity is a telephone number, it should be represented either by a Tel URI or by a SIP URI that includes a "user=phone" URI parameter. For global numbers, these must follow the E.164 format, ensuring interoperability and standardization in IMS networks.