VoIP Cookbook: Analog Telephone Adapter for connection to PSTN

It is obvious that making both VoIP and PSTN to coexist is difficult, particularly when you are seeking to minimize your telecommunication spending by choosing either one of them. So this section will help you understand how to keep your PSTN line by using a trunking equipment to be connected to PSTN. Generally, the equipments available in the market have one FXO or two. Coincidentally, the Linksys SPA400, the equipment we use as an example, has four FXOs and is relatively less expensive. The interest here is that you want to have VoIP be connected to your conventional phone, either to a PABX or directly to your PSTN network. This usually does not require you to obtain a license, unless you intend to become a provider with commercial interest.

So for your purpose, you need to have an Analog Telephone Adapter (ATA), which can be connected to the cable of your PSTN cable. In VoIP, this is often called FXO (usually labeled as Line). The physical condition of this interface is similar to that of RJ-11 (conventional telephone jack). The difference is that in in RJ-11-FXO line, there is no voltage, something that is there when you have a RJ-11 Phone. With FXO, you can your VoIP be connected to PSTN and PABX extensions. And ATA phone can be connected to both a regular phone and PABX CO.

In short, there are two type of RJ11 connections in ATA, namely,

FXO to be connected to PSTN / Telco line / PABX extension.
FXS to be connected to Telephone line / FAX.

When connecting ATA to PSTN line, make sure that you do not connect it to the wrong plug. If you did, the PSTN's voltage, which usually is around 48 V, would collide with that of the ATA phone. This will damage your ATA equipment. So prior to connecting them, you have to set your ATA equipment so it recognizes whether the voltage in place is 48 V or 24 V.

VoIP Cookbook: Analog Telephone Adapter for connection to PSTN

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