I figure I'd just throw this out there just for the heck of it...
For what it's worth, I use a Cisco ISR 2851 as an FXS and FXO gateway in my home lab and am very happy with it. You can nab them pretty cheap on eBay (either a 2851 or 2821 would work). You'd just need either two VIC2-4FXO cards or for the more adventurous an EVM-HD-8FXS/DID with two EM-HDA-6FXO daughter cards and something to terminate the RJ21 connector with (like a 66 block or a breakout cable)...the former is likely easier though :grinning_face_with_smiling_eyes: