What is a SIP trunk?

The word trunk is inherited from copper. A trunk was the bundle of lines running from a building to the exchange, and a business bought as many as it needed to handle simultaneous calls.

The modern version carries the same idea over your data connection. You buy capacity rather than cable.

Providers sell it two ways. Some charge per concurrent channel, so ten channels means ten calls at once. Others sell unmetered capacity and charge for the minutes.

Sizing catches people out. Count how many calls happen at the same time during your busiest hour, not how many calls you get in a day.

Suppose a veterinary clinic takes 180 calls a day. The call log shows the busiest hour is 9:00 to 10:00, with 34 calls averaging three and a half minutes each.

Multiply those and you get 119 call-minutes packed into a 60-minute hour. Divide by 60 and the answer is just under two, meaning that on average two calls are in progress at any moment during that hour.

Average is the trap. Calls bunch up, so at some point in that hour five people will be on the phone at once. Telecom engineers use traffic tables for this. For a load of two, the tables say six channels will turn away roughly one caller in a hundred. Seven gets you to about one in three hundred, and five would block about one in thirty.

So this clinic wants six or seven channels for 180 calls a day. The person who guessed twenty from the daily total would have paid for thirteen channels that never carry a call.

A channel is one call in progress. Your phone number, by contrast, is just an address that points at your trunk. The two aren't linked, and mixing them up is the most common confusion when people read a quote.

You can have fifty phone numbers, one for every staff member and every ad campaign, all arriving over the same six channels. All that six limits is how many conversations happen at once.

When every channel is busy, the next caller hears a busy signal or a carrier recording, depending on the provider. Nothing appears in your own phone system's log, because the call never reached it. Ask your provider whether they can report blocked calls, since that's the only place the evidence exists.

Outbound calls use the same pool. A staff member making follow-up calls at 9:30 is taking a channel away from inbound callers during your peak.

GreetKeeper does not sell or need one. Calls arrive by forwarding, so your existing arrangement with your carrier stays exactly as it is.

If you already run trunks, the assistant becomes one more destination behind them.

We bill per answered call rather than per channel or per minute, which is a different unit from the one your trunk provider uses.