How does an ATA work?
If I buy an ATA and connect it to an existing router, will I get a dial tone right away?
How do I know the ATA's IP address?
Will I be able to talk if I can connect to another ATA?

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Most people think of an analog telephony adapter (ATA) as a device with one or more analog FXS ports, which is used to connect to analog phones or fax machines, and an Ethernet port. For this discussion, that will be the assumption. We'll also assume the protocol being used is SIP -- the most popular protocol for Voice over IP today. Typically, before an ATA will provide dial tone to an analog device connected to it, the Ethernet port must be connected and the ATA must be able to register the devices attached to it with a SIP server. Once the devices are registered, then the dial tone is typically provided. This is the most common way an ATA is used.

You can find out the ATA's IP address by dialing some DTMF code, specific to the ATA manufacturer, from an attached phone. The ATA will then read the IP address to you audibly, along with the subnet mask and default gateway information -- typically obtained through DHCP.

If there is no SIP server or service provider, and you want to call from one analog phone to another phone on a separate ATA, you can typically disable registration on the ATA and have it send all calls to a specified IP address. This requires setting up fixed IP addresses on each ATA.

This was first published in March 2006