How Many Phones — and How Many Calls — Can My Network Carry?

The right question: not "how many phones," but "how many calls"

The common question is "How many phones can I connect to my internet?" The honest answer is: almost as many as you want. An idle phone uses almost nothing — a small "I'm here" message to the server once every few tens of seconds, which is less than a kilobit per second. Even a hundred idle phones won't be felt on the line.

What uses the line is an active call. Each call is a constant stream of voice in both directions, for as long as it lasts. So the right question is: how many calls are in progress at the same time during the peak hour?

In an ordinary office, not everyone talks at the same moment. In the busiest hour, usually between a quarter and half of the employees are on calls. In a sales or service call center, by contrast, almost all the agents talk all the time. This difference is the basis of the whole calculation.

There are three possible "bottlenecks," and you check all three: the internet line (mainly the upload), the switch (the number of ports and the amount of power for PoE), and the router (how many connections and how much load it can hold). We'll go through them in order.

Step 1: how many simultaneous calls you have

It is better not to guess this number. On an existing PBX, the call history shows how many calls overlapped in time during the busy hour of a regular week. If there is no data, you can estimate by type of activity:

Don't forget calls that don't reach an agent: waiting in a queue, listening to an IVR menu, leaving a message. On a cloud PBX, these all happen on the server and don't pass through the office line at all. Only a call answered at an extension in the office uses the office line.

A rule of caution: add about 20%–30% to the result for growth and unusual days — before a holiday, at the end of the month, on a day when everyone calls.

Step 2: how much each call uses — by codec

The codec is the method by which voice is compressed for transmission over the network. Each codec uses a different amount, and on top of the voice itself there is overhead — the headers of every packet, which in small voice packets are a large part of the weight. The numbers here include the overhead, per direction:

For a simple, safe calculation, assume 100 kilobit per second per call, per direction. This covers G.711 and G.722, and leaves a margin for the economical codecs. Ten simultaneous calls = about one megabit of upload and one megabit of download.

Step 3: upload versus download

A call is symmetric: your voice goes out (upload), and the other side's voice comes in (download), in exactly the same amount. So you need the same bandwidth in both directions.

The problem is that most internet packages are not symmetric: the download is large, and the upload is much smaller. On copper and cable lines the upload is sometimes a tenth of the download, or less. On optical fiber it is usually much larger, and sometimes equal to the download.

So almost always the upload is what decides. A "200 megabit" line with 5 megabit upload is limited in calls by the 5, not by the 200. And since backups, emails with attachments and video calls also use the upload, it runs out first.

Step 4: the table — simultaneous calls by upload speed

The calculation: the upload speed divided by 100 kilobit per call, and then half of that — a safe 50% margin for everything else using the line. If calls are the only thing on the line (for example a separate line for the phones), a smaller margin can be used.

The table shows something reassuring: for most offices, even a modest line is enough for calls. The real problem is almost never how many megabits you have, but what else is running on the line at that moment — and whether the line is stable. The next step is devoted to that.

The short calculator, for anyone who wants to calculate on their own in a minute:

1. How many simultaneous calls at peak hour? (plus 20%–30% for growth)
2. Times 0.1 megabit = the upload the calls need.
3. Plus everything else that uses the upload at the same time: computers, video, cloud cameras.
4. Times 2 — a safe margin.
5. Compare with the upload speed actually measured, at a busy hour, from a computer on a cable.

Step 5: who else shares the line

The phones are not alone. Every device in the office that sends data to the internet competes with them for the same upload. The estimates here are general and depend on settings, but they give an order of magnitude:

The last row is the most common reason for complaints. A backup or sync isn't satisfied with "a little" — it takes all the upload there is, and in those minutes calls are cut off. So they are limited in settings, moved to nighttime, or calls are given priority using QoS.

Step 6: the switch — ports, speed and PoE

Every desk phone is connected to a switch by cable. The first question is simple: how many ports. Count phones, access points, cameras, printers and computers that are not connected through the phone, and add about 20% for growth. Switches are sold in fixed sizes — 8, 16, 24, 48 ports.

In terms of speed, calls don't load the switch at all: even a simple switch with 100 megabit per port carries hundreds of calls. Still, a 1,000 megabit (Gigabit) switch is better, because computers connected through the phone will benefit from it.

The question that really matters is PoE — the power the switch delivers to the phones over the cable. Every PoE switch has a total power budget, in watts. If the total consumption of the devices exceeds it, some of the ports simply won't get power, and phones will turn off or restart themselves.

PoE budget — how to calculate it

Add up the consumption of all the devices that receive power from the switch, and compare it with its budget. Common estimates:

The basic PoE standard (802.3af) supplies up to about 15 watts per port, and the enhanced standard (802.3at, "PoE+") up to about 30 watts. It is needed for powerful access points and some cameras. For phones, the basic standard is almost always enough.

A rule of thumb: don't plan to use more than 80% of the switch's power budget. A switch running right at its limit overheats, and every extra phone may "knock out" another port.

Step 7: The router — connections, load and settings

The router is the gateway of the whole office to the internet, and every packet of every call passes through it. Three things decide whether it can handle the load:

• The number of open connections — the router keeps track of every active connection (for NAT). Each phone holds a permanent connection to the server, each call opens more, and every computer opens dozens or hundreds of connections while browsing. Cheap home routers are sometimes limited to a few thousand connections, and in a busy office they fill up.
• Processing power — many small voice packets burden the router's processor more than one large download. In a call center with dozens of calls, this is noticeable.
• Correct settings — SIP ALG off, UDP connection hold time long enough, and QoS if the line is shared.

One more word about Wi-Fi in the calculation: a single access point can hold dozens of connected devices, but how many simultaneous calls it carries comfortably is a different matter — all the devices share the same airwaves. In practice, you plan how many softphones are active per access point, and add access points as the number of people on calls grows. Fixed stations are connected by cable, and then this question simply doesn't arise.

Example 1: An office with 10 stations

An accounting office (fictitious name: "Cheshbon Tzedek") — 10 employees, a desk phone and a computer for each employee, a printer, and one access point for laptops. Internet line: 100 Mbps download, 10 Mbps upload.

The computers connect through the computer port on the back of the phone, so one cable per desk is enough. The conclusion: the existing line is completely sufficient. What matters here is that the computers' backup doesn't run during working hours.

Example 2: A yeshiva with 40 phones

A yeshiva (fictitious name: "Yeshivas Ohel Torah") in two buildings: 40 phones — in the office, in the administration, in the rebbeim's rooms, in the dormitory and in the kitchen. Four access points for staff, and six cameras that record to a local recorder. Line: 200 Mbps download, 20 Mbps upload.

Here the line isn't the problem at all — ten percent of it is enough for the calls. The challenge is the infrastructure: a 48-port PoE switch with a budget that leaves headroom above 260 watts, and a separate small switch for the cameras. Because a network cable is limited to about 100 meters, the second building gets its own switch, connected to the main one by cable or by fiber.

If the cameras were streaming to the cloud, the picture would be different: six cameras × 3 Mbps = 18 Mbps, almost all of the upload. This is a good example of how the question "how many phones" sometimes hides the real consumer of the line.

Example 3: A call center with 25 agents

A service call center (fictitious name: "Emun Call Center") — 25 agents with a phone and headset, a team leader, and a computer for each agent with a customer system in the browser. Almost all the agents are on calls all the time.

In a call center, calls are the heart of the business, so the recommendations are stricter: a fiber line with large upload, QoS that gives calls absolute priority, a business router, a separate network for phones (VLAN), a UPS for the cabinet, and a backup line that kicks in when the main one goes down.

Two reassuring notes: in a cloud phone system, call recording and the team leader's listening-in are usually done on the server, and they add no traffic to the call center's line. Also, callers waiting in the queue don't use the line — only answered calls do.