I have been using AudioTX for years, connecting via bridge to ISDN network. But with an ISDN line and a USB adapter, it can replace the Telos Zephyr or CDQ Musicam codec. A used laptop or cheap netbook will work just fine. Digifon's Dave Immer is a dealer and will help you set it up and be your tech support. I have found it to be totally bulletproof over many years. And connecting over IP (internet) it requires about half the upstream bandwidth that Source-Connect requires.
Can't go wrong here. - Beau Weaver
Thanks for your reply, Beau. I'm ballparking $1500 for netbook, ISDN adapter, and AudioTX to get into this. (Already have an MBox to dedicate to it.) That's a heckuva lot cheaper than a Zephyr. :) Happy New Year!
You'll also need an NT1 terminal adaptor. It is cheaper than a new Zephyr, but used Zephyrs show up all the time for less than $1500, and they can be much easier to setup. YMMV, but don't skip on Dave Immer's help when setting up ATX with ISDN.