Question: In an IP Multimedia Subsystem (IMS) scenario where both IMS signaling and media traffic are anchored in the Visited network's EPS/5GS, what action does the User Equipment (UE) perform to initiate communication, according to standard procedures?
Option 1: UE directly establishes a connection with the Serving Call Session Control Function (S-CSCF)
Option 2: UE performs a DNS query for the nearest Serving Gateway (SGW)
Option 3: UE performs a P-CSCF (Proxy-Call Session Control Function) discovery
Option 4: UE initiates media transfer without signaling setup
Explanation: In IMS, before initiating a call or session, the User Equipment (UE) needs to discover the Proxy-CSCF (P-CSCF), which is the entry point within the IMS for mobile devices. This discovery is crucial for routing signaling messages correctly and ensuring that media traffic is properly anchored in the network. The P-CSCF discovery process allows the UE to identify the appropriate P-CSCF to use, facilitating further communication setup with the Serving CSCF (S-CSCF) and other network elements.