SIP • Last Updated 9/29/2023

SIP Trunk 101: What is Call Origination?

Learn about call origination with Telnyx: what it is, how it works, and what to look for in a provider.

By Michael Bratschi

What is VoIP Call Origination

Let’s get back to basics here: Knowing how voice telephony call origination works is very simple – someone picks up their device or soft phone, enters your phone number, and you pick up the phone. In essence, call or voice origination is an incoming call. Of course, the actual “how” is a bit more complicated.

A Quick POTS History

With plain old telephone service (POTS) or more commonly known dial-up circuit architecture, the user must “seize” the line or circuit before a call can be established. Once this has been initiated, the caller will hear a dial tone and can begin entering the phone number.

Depending on the distance between the caller and callee, a Central Office that contains Class-5 or End Office Switching equipment then routes the call to the recipient’s Central Office through a series of Class-4 or Tandem switches.

Class 4 or Tandem Switch Diagram

This entire infrastructure is typically referred to the Public Switched Telephone Network (PSTN). Eventually, the receiver will ring if the recipient’s line is not seized and if it is, the call initiator will get a busy signal.

The Call Origination Process

Today, VoIP calls use packet switching technology, which digitizes the voice and sends the data to the recipient. The call origination then is the point where the call started - typically on the PSTN –as it is transferred from the Class 4 Tandem Switch by an Internet Telephone Service Provider (ITSP) over the Internet to a destination point. More specifically, the media is transferred via a Session Initiation Protocol trunk or SIP trunk – basically digital telephone lines that VoIP providers use to connect your private branch exchange (PBX) or telephone system.

Choosing the Right Call Origination Provider

The key to choosing the right VoIP origination provider is dependent on a few factors and caveats:


Price is a huge consideration that’s typically always at the top of the list. Different providers have different price points depending on your needs and the services that you’re looking for. There are also pricing structure differences as next generation VoIP carriers provide a la carte pricing versus more traditional providers that will typically make you sign a contract.


Functionality and interoperability could also make or break your decision when it comes to choosing a voice origination carrier. If receiving calls is an immediate need for your organization, how quickly will you be able to integrate your existing PBX with the VoIP provider that you’re considering? And what about features and usage? Make sure you’re trying out their platform first before making any final commitments. Check out how intuitive the system is and play around with the features to really gain a feel of how useful the platform is going to be for you.

Overall Quality

Take the time to really find out how each voice origination providers differ and which one can best accommodate your needs. 24/7 support, for instance, may be something that you absolutely must have, and not everyone can offer that service. And beyond support, look into the actual network that the provider offers that will best suit your inbound needs – do they have a private backbone that will take your media off the public internet? Will they have security features that will protect your data?  

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