SIP Trunking With a VoIP Gateway

July 16, 2010 by Brian Hyrek

It’s certainly true that IP telephony offers a much more robust telephony experience than traditional analog telephony. However that robust experience often comes with an upfront expense.

So what if you’re not financially ready to invest in a VoIP PBX?

You’ve still got options.

VoIP Service providers most commonly offer SIP Trunking in order to allow you to make VoIP calls using your phone system – either analog or VoIP. If you’ve never heard of a SIP Trunk a good way to think of it is as a phone call routed over the IP infrastructure of a carrier using VoIP technology. Today SIP Trunking is the standard transport method for VoIP calling.

For your business utilizing SIP Trunking means you no longer no longer need a PSTN connection or costly PRI/BRI’s. Electing to take advantage of SIP Trunking affords the opportunity for smart and cost effective solutions such as only requiring one connection for both data and voice.

After a bit of research, you may have decided on a SIP Trunking provider that best suits your needs.

This leads us to how exactly you leverage SIP Trunking with your existing PBX.

For that you’ll need an FXS VoIP gateway.

SIP Trunks can be brought in to an Analog or Legacy PBX via a FXS gateway. In a prior blog, I had touched upon FXS and skimmed the possibilities of an FXS gateway. FXS, Foreign Exchange Station, is a telephone interface which supplies battery power, provides dialtone, and generates ringing voltage. FXS devices must connect to an FXO device such as your Analog PBX.

Connecting an FXS Gateway to Analog PBX

Take a look at any Analog PBX and you will notice FXO ports on the actual PBX. An FXS gateway will be connect to these ports in a 1 to 1 type setting. Meaning, If you have 8 FXO ports on your Analog PBX, you will require an 8 port FXS gateway to connect to the SIP trunks.

Take a look at this diagram on SIP Trunking via a VoIP Gateway:

FXS Gateways come in a range of sizes from 2-24 port to suit businesses of all sizes. They can also be used in conjunction with one another to scale even higher.

All in all, it is possible to “save” your recent investment in an analog PBX or further your communications experience with SIP Trunking while not being able to financially support an IP PBX.


  • Mitchell Bendzans

    Hi Brian. Need some help… We have been having an awful time with our PRI circuits in regard to service interruptions… We have 2 carriers and both have presented challenges. Our phone system is a Nortel CS1K; not ready to replace it yet, so I’m looking to see if there is an option to eliminate the use of PRI’s by employing a SIP gateway (use the internet). Hoping that you can provide some assistance/guidance.

    • Hi Mitchell,

      Thanks for reaching out to us! Brian is out of the office today so I have assigned another rep to you. She will be contacting you shortly to help out. Thank you!


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