[Serusers] Looking for FXO or FXS ATA

We're an ISP on the point of deploying VoIP for SMB and SOHO, and we want to implement some solution that allow an employee to pick up the the phone, and dial some prefix to take our line and make long distance calls via Internet. But also keep the capability to dial some another prefix and make a local phone call via traditional carrier (directly to PSTN). I understand the ATA we decide to work with, must to have few FXO ports to connect with FXS PBX ports. May be it could be done with ATA FXS ports connected to PBX FXO ports. Is that right? If so... what type of connection do you recommend? And which equipment? Thank you very much in advance. Sebastian Milioto Telecommunication Engineer IM: sebamilioto(a)hotmail.com Mobile: +54 3571 15 563 6099 E-mail: smilioto(a)gmail.com