This is a somewhat unorthodox answer, but for truly cost effective and reliable FXO functionality, you cannot beat (used/unlocked) Linksys RTP300.  It has a 4 port switch built in which may or may not be usefull.  But for 2 FXO ports you can’t beat it for around $10-$12 on EBAY (or new around $50 - $75).

I have used these for home adapters for my house and several others for years.  They are quite reliable.  They support NATing just fine.

Just something to consider.

From: freeswitch-users-bounces@lists.freeswitch.org [mailto:freeswitch-users-bounces@lists.freeswitch.org] On Behalf Of Blake Priddy
Sent: Thursday, December 19, 2013 9:42 PM
To: FreeSWITCH Users Help
Subject: [Freeswitch-users] FXO Card or Gateway

Recommendation? Cost effective and reliable please :)