What is SIP?

SIP does the negotiating. It rings the other end, agrees how the audio will be encoded, and hangs up afterwards.

The audio travels separately, usually over RTP. That split is why a call can connect perfectly and still have terrible sound.

A SIP trunk is the business version: a connection between your phone system and a provider that carries a set number of simultaneous calls.

Here's what happens in the second after someone dials a SIP-connected business line. The caller's carrier sends your provider a message called INVITE. It says who's calling, which number they want, and which audio formats the caller's side can handle. Your side answers "100 Trying" and then "180 Ringing", and that second message is what makes the caller hear a ring tone.

When the phone is picked up, your side sends "200 OK" with its own list of audio formats, and the caller's side confirms with ACK. Only now does audio start flowing. It travels on a separate path, in whichever format both lists had in common. At the end, whoever hangs up first sends BYE. All of it is plain text, which is why a phone engineer can read a failed call like a transcript and tell you in a minute whether the problem was the signaling or the sound.

First, how many channels does your account have? You already know that number caps simultaneous calls, so ask the follow-up too: does a forwarded call use one channel or two? On some systems a call that comes in and gets sent out again occupies two until it ends.

Second, what happens when your equipment is unreachable? A power cut or a dead router takes your phones offline. Most providers let you name a failover number where calls go automatically in that case, and it's rarely set unless you ask.

Third, if calls connect but you sometimes hear nothing, or only one side can hear, ask about a router setting called SIP ALG. It's meant to help SIP traffic through a firewall, it frequently mangles the messages instead, and many VoIP providers tell customers to switch it off.

You do not configure SIP to use GreetKeeper. Forwarding from your existing number is a carrier setting, which is why setup takes minutes rather than an engineering project.

If you already run a SIP trunk, the number worth checking is its channel count, because that caps how many calls can arrive at once no matter what answers them.

The assistant answers concurrent calls in parallel, so on the software side there is no queue and no hold music.