---
title: "CT Cloud SIP - Signaling Basics"
slug: "internal-ct-cloud-sip-signaling-basics"
tags: ["calltower", "internal", "VoIP Monitor", "Call Flow", "Sip Signaling", "CT Cloud Solutions"]
updated: 2026-09-18T16:14:20Z
published: 2026-09-18T16:14:20Z
canonical: "learn.calltower.com/internal-ct-cloud-sip-signaling-basics"
---

> ## Documentation Index
> Fetch the complete documentation index at: https://learn.calltower.com/llms.txt
> Use this file to discover all available pages before exploring further.

# CT Cloud SIP - Signaling Basics

**Last updated:** Mar 9, 2021, 5:17 PM by mlym@calltower.com

## Overview

This article will go over and explain the SIP Signaling basics using VoIP Monitor.

### Sip Signaling Basics:

In a call flow you have your basic sip signaling. Shown below:

INVITE: This is what initiates the call set up.

![Invite.png](https://cdn.us.document360.io/4662144b-ab1f-4796-8b7b-e9f9c609894f/Images/Documentation/Invite.png?sv=2026-02-06&spr=https&st=2026-11-05T00%3A12%3A21Z&se=2026-11-05T01%3A12%3A21Z&sr=c&sp=r&sig=tlLRe%2FF2w9hpSoSnGJqMmtGMUqU75hEeCIgRmFcDxaM%3D)

100 TRYING: This is the receiving end searching for the user/number.

![100trying.png](https://cdn.us.document360.io/4662144b-ab1f-4796-8b7b-e9f9c609894f/Images/Documentation/100trying.png?sv=2026-02-06&spr=https&st=2026-11-05T00%3A12%3A21Z&se=2026-11-05T01%3A12%3A21Z&sr=c&sp=r&sig=tlLRe%2FF2w9hpSoSnGJqMmtGMUqU75hEeCIgRmFcDxaM%3D)

180/183: This is the receiving end establishing a connection and sending ringing.

![180ringing.png](https://cdn.us.document360.io/4662144b-ab1f-4796-8b7b-e9f9c609894f/Images/Documentation/180ringing.png?sv=2026-02-06&spr=https&st=2026-11-05T00%3A12%3A21Z&se=2026-11-05T01%3A12%3A21Z&sr=c&sp=r&sig=tlLRe%2FF2w9hpSoSnGJqMmtGMUqU75hEeCIgRmFcDxaM%3D)

200 OK: This is the receiving end picking up the call and choosing which codecs and audio ports to use on the call.

![200OK.png](https://cdn.us.document360.io/4662144b-ab1f-4796-8b7b-e9f9c609894f/Images/Documentation/200OK.png?sv=2026-02-06&spr=https&st=2026-11-05T00%3A12%3A21Z&se=2026-11-05T01%3A12%3A21Z&sr=c&sp=r&sig=tlLRe%2FF2w9hpSoSnGJqMmtGMUqU75hEeCIgRmFcDxaM%3D)

ACK: this is the originating end agreeing to the 200 ok response and completing SDP setup.

![ACK.png](https://cdn.us.document360.io/4662144b-ab1f-4796-8b7b-e9f9c609894f/Images/Documentation/ACK.png?sv=2026-02-06&spr=https&st=2026-11-05T00%3A12%3A21Z&se=2026-11-05T01%3A12%3A21Z&sr=c&sp=r&sig=tlLRe%2FF2w9hpSoSnGJqMmtGMUqU75hEeCIgRmFcDxaM%3D)

RTP: When you see RTP this is the Audio on the call, you want to see this is both directions like shown below.

![RTP.png](https://cdn.us.document360.io/4662144b-ab1f-4796-8b7b-e9f9c609894f/Images/Documentation/RTP.png?sv=2026-02-06&spr=https&st=2026-11-05T00%3A12%3A21Z&se=2026-11-05T01%3A12%3A21Z&sr=c&sp=r&sig=tlLRe%2FF2w9hpSoSnGJqMmtGMUqU75hEeCIgRmFcDxaM%3D)

Session Initiation Protocol, is the technology that starts, manages, and ends internet-based phone calls and video meetings between people and devices.