When a PSTN user dials SIP user 5000 under U1981 through the PRA trunk, what is the process on U1981?

Question

Options

• AIntra-office prefix 5 -> EID -> Number 5000 -> IP address of user 5000
• BIntra-office prefix 5 -> EID -> IP address of user 5000
• CIntra-office prefix 5 -> Number 5000 -> EID -> IP address of user 5000
• DIntra-office prefix 5 -> Number 5000 -> IP address of user 5000

• A
  15% (7)
• B
  9% (4)
• C
  72% (34)
• D
  4% (2)

Explanation

Option C is correct because on the U1981, incoming PSTN calls via the PRA trunk follow a sequential resolution process: the system first applies the intra-office prefix (5) to classify the call as internal, then identifies the dialed number (5000), maps it to an EID (Endpoint ID) in the routing table, and finally resolves the EID to the SIP user's IP address for delivery. The EID is a mandatory intermediate step - it acts as an internal identifier that bridges the E.164-style number to a network endpoint, and you cannot skip it.

Why the distractors fail:

• A places EID before Number 5000 - impossible, since the system must know which number it's routing before it can look up an EID.
• B omits Number 5000 entirely - the number must be extracted from the call signaling before any database lookup occurs.
• D jumps directly from Number 5000 to IP address - this skips the EID lookup, which is the required indirection layer in U1981's architecture for mapping numbers to SIP endpoints.

Memory tip: Think "Prefix → Number → EID → IP" as a two-hop address resolution, like a phone book that first gives you a code (EID), then you look up the code to get the actual address (IP) - the number always comes before the code, not after.

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