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Back in the day, just a few short years ago, we thought about and worked with telephone equipment with the understanding that telephony was a dedicated circuit carrying your voice from Point A to Point B. These circuits were built for reliability and predictability. I can still remember the days when telephony was Tip & Ring, Ground Start, Loop Start, T1s, and PRIs. In the Call Center arena, we controlled a physical phone using an RS-232 serial cable connected to a board installed in the phone. Today, when we think of “telephony,” it is so much more than just voice. In fact, voice is just one real-time media stream among many (video, screen share, messaging, presence), and it often rides across the same IP networks that deliver our email, streaming, and cloud apps. Voice is no longer just a call path—it’s a workflow trigger that can launch messaging, analytics, transcription, routing logic, and even AI-driven automation.
[Related Case Study: NorthBay Health | Addressing Pain Points with Upgraded Call Center Software]
At the center of this shift is VoIP (Voice over IP): turning voice into packets, sending them over IP networks, and reassembling them in real time. Two major protocol families helped shape this world: H.323 and SIP. These are the “call setup” languages that tell callers how to find each other, negotiate capabilities, and start (and end) real-time media streams like voice and video. Understanding these protocols explains a lot about how modern calling works, from enterprise Private Branch Exchanges (PBXs) to video meetings to call centers.
What “Modern Telephony” Actually Means
Modern telephony, over time has slowly reinvented itself. The PBX and carrier world was a stand-alone, tightly controlled world containing physical equipment that ran on wiring separate from all other wiring. This physical equipment and stand-alone network no longer define today’s telephony world. The landscape today is defined more by the software and protocols that set up, manage, and tear down these real-time calls. If you’ve ever joined a Zoom call, placed a Teams call, used a softphone, or connected to a call center agent, you’ve interacted (directly or indirectly) with systems that do three key things:
- Signaling: “Who are you calling? Where are they? Can we connect?”
- Media transport: “Here are the voice/video data—keep them flowing smoothly.”
- Control and services: “Put the caller on hold, transfer, record, conference, secure the call.”
The main takeaway here is that signaling and media are typically separate now. Signaling sets up the call; media carries the audio/video.
A Quick Foundation: Signaling vs. Media
Let’s do a quick review of today’s modern VoIP data environment:
First, we have the signaling protocols that establish sessions, figure out capabilities, route calls, and place calls on hold, transfer, or conference calls. The most common protocols used for this today are Session Initiation Protocol (SIP) or the older H.323 protocol.
Next, we have the media protocols that carry the audio and video. These rules rely on a real-time transport protocol (RTP).
Both SIP and H.323 typically use RTP for media transport once the call is established. Here, we are not so concerned about the actual voice traffic but more interested in how this traffic will live in our network environment and what we need to do to deploy and troubleshoot this VoIP environment.
H.323: The Enterprise Workhorse That Came Early
A brief history of H.323 reminds us that this suite of ITU-T standards was originally designed for multimedia communications riding on standard data networks. These are the networks that include servers and PCs (often competing with other IP traffic), network latency, and unpredictable conditions. The H.323 family is therefore designed to handle a lot of complexity.
Let’s briefly discuss how H.323 works, at least from a conceptual perspective, and why this was originally a popular protocol. H.323 is not a single protocol but a collection of protocols that includes geeky numbers such as H.225 for signaling, H.245 for media, RAS (Registration, Admission, and Status), and RTP for transporting the actual audio and video.
If you’ve heard the terms gatekeeper, MCU, or endpoint in old-school video systems, that’s classic H.323 territory.
H.323 was popular because it was an early standard for voice and video conferencing. It had a robust feature set that was good for enterprise environments, and it clearly defined roles for the network components.
Unfortunately, it was also considered a “heavy” protocol with a lot of moving parts. It has a tough time getting through firewalls and dealing with Newark Address Traversal (NAT) needs. Bottom line is that it is way more complex than later alternatives.
H.323 will still appear in enterprise video conferencing deployments, some telecom interconnection scenarios, and environments where older long-lived investments and infrastructure still exist.
SIP: The Internet-Native Language of Calling
Introducing SIP (Session Initiation Protocol). A protocol designed with an eye on the Internet. This means it includes text-based messages, extensibility, and plays well with proxy routing, firewalls, and other web-like infrastructure conditions.
SIP is a protocol that can find users and devices, set up sessions, modify those sessions, and cleanly end those sessions.
SIP has become the “winner” or predominant protocol for modern VoIP because it is simple and readable with its HTTP-like text. It is easier to look at logs and captured data, making troubleshooting much less complicated. SIP supports a wide variety of devices and scenarios, including desk phones, softphones, carriers, WebRTC, conferencing, and many more. Plus, as IP telephony grows, more vendors, carriers, and open-source projects adopt SIP.
We see this with organizations that no longer want proprietary hardware for desk phones or want to move to softphones. We see this with hosted PBX and UCaaS (Unified Communications as a Service) solutions, along with carrier trunks moving to SIP. Call Centers of all types can easily integrate with PBXs and UCaaS systems utilizing SIP.
SIP vs. H.323: A Practical Comparison
When we briefly compare H.323 with SIP, we find several areas that mean something to us in the real world. From an architecture perspective, H.323 is a standard with multiple sub-protocols rich in features but often considered a heavier solution. Contrarily, SIP is a leaner protocol that takes advantage of companion standards (like SDP for media negotiation).
SIP is everywhere. Enterprises, carriers, cloud solutions, consumer applications, and call centers all use SIP today, while H.323 is mostly seen in enterprise environments that use it for legacy video.
When it comes to troubleshooting, H.323, with its multiple channels, requires subject-matter expertise for deeper analytics, while SIP, as a text-based solution, lowers this requirement.
Plus, as SIP is adopted by more vendors as a standard, firewall and NAT (Network Address Translation) traversal become less of a headache because of all the tools available to address these challenges.
The Supporting Cast: Codecs, RTP, and the “Quality” of Calls
We need to spend a minute on quality because even with good signaling, the quality of the actual media can depend on many things, such as the Codecs used, the RTP used, Jitter, quality of service (QoS), noise, and echo. Again, signaling can help identify what is possible, but the media layers decide what is the best at the moment based on bandwidth and latency, for example.
Standard Codecs, which compress the data, include the classic G.711 and G.729. Opus is a newer, high-quality, adaptable format quickly gaining ground in modern real-time applications.
RTP formats carry the data in real-time, while jitter buffers smooth everything out so all the data arrives at the same time. QoS helps prioritize voice and video on networks, while echo cancellation and noise suppression are increasingly being taken care of by the software endpoints.
Security and the Role of Session Border Controllers
We hear about SBCs (Session Border Controllers) a lot these days. That’s because these guys sit at the edge of the VoIP network and make sure all is well. They help control and secure signaling and media. They act as arbitrators between vendors, making sure there is good interoperability. They also help with NAT issues and assist in enforcing any policies, say for fraud, as well as encryption of the signaling and media, which is often TLS.
So, they are kind of a big deal and deserve some attention even if we typically don’t hear about them on a day-to-day basis.
So Where Is Modern Telephony Going?
The three main directions that help illustrate where telephony is going can be seen in the Cloud-first calling (UCaaS and CCaaS) initiative, which is increasingly becoming available where voice is no longer a box in a closet but is now a service managed in the cloud.
The second is that telephony is no longer a separate component from meetings and messaging. These platforms are unified with voice as a part within that unified platform.
And third browser and native applications integrating calling by taking advantage of protocols such as WebRTC and cloud communications platforms.
Final Takeaway: SIP Is the Default, H.323 Is Legacy
Simply put, SIP is the signaling protocol of the modern VoIP environment, whether in an enterprise, carrier, or cloud communications environment. H.323 is legacy and still utilized in certain enterprise video and long-lived telecom deployments. It helps illustrate from a historical perspective how communication has matured over the years.
Understanding both helps us navigate interoperability issues, troubleshoot weird call setup problems, and make sense of why modern systems look the way they do. Talk to us about your organization’s pain points. Together, we can ensure the right data reaches the right people at the right time, to support value-based care and better patient outcomes. Amtelco’s guiding principle is to deliver cutting-edge technologies built on a tradition of innovation, reliability, and care. In practice, that means providing the most modern integrations required from health systems.
