Signaling Server" is the generic name for a hardware platform which can run multiple different software applications. What you would have historically called an "IP Line" application is one of those - and is the ones to which the IP Phones register. "Element Manager" is another. "H.323 Gatekeeper","SIP Proxy Service (SPS)", "Network Routing Service (NRS)", "Directory (Personal/Callers/Redial)" are others that all run on a "Signaling Server". With 5.0, this can even be consolidated onto a card within the system instead of being an external server.