If you want to create a VOIP to PSTN gateway and only have an
ATA with an FXS port, you can use an FXS-FXO converter connect the FXS port to a PSTN phone line.

Internet------ATA-----FXS-FX0 converter------PSTN phone line

This has been a popular option for creating a low cost PSTN gateway because of the lack of low cost ATAs with a FXO port. This option will probably continue to be popular even with the advent of the Sipura 3000 given the increasing numbers of VoIP service providers who provide users with their own nominated preconfigured brand of FXS gateway or ATA.

If you are using an FXS-FXO converter with a Sipura ATA then see this
mailing list posting.

Sources of FXS-FXO converters

• Auerswald A-Box
• BroadTel FXS to FXO Port Converters: FXS to FXO could be 1:1, 2:1, 3:1,4:1... and many others.
• ICS France FXS to FXO Port Converter . 4-48 port FXS/FXO Gateway,
• PCPhoneline.com - VPC1000 FXS to FXO Port Converter - Only $39
• Realtone Technology FXS to FXO converter VoIP Gateway support 4,8,16,24,32,48,72, up to 96 ports, compatible with Asterisk,3CX
• SIPCPE NEW FX-300 Analog Call Director, a converter and 2 line switch in one unit with auto busy tone detection, volume boost, echo cancellation and CID authentication. sales@sipcpe.com SIPCPE also has a new Cellular (GSM) to Analog or VOIP converter/switch called the FX-300 GSM Call Director.
• Welltech 2/4-port FXS, 2/4/6-port FXO, 1/2 FXS+FXO, IAD 161/162, IP phone
• PBXEQ
• X100P.com announces STOC-FX, Professional Grade FXS to FXO Converter for All VoIP ATAs and Asterisk FXS Ports