We maintain an official compatibility list where we enumerate devices that have been tested and proven to work reliably with our service. A 'Certified' device means we have worked directly with the manufacturer and a formal, official interop test has been done between our companies. Devices we have tested in our own labs without direct cooperation from the manufactured are listed in our 'Approved' category. You can expect our service to work flawlessly with any of these interop approved products.
Our goal is to be interoperable with all T.38 implementations, and we enjoy a challenge, so if you have a product (or even better, if you make a product) that you don't see here, send us a message with the specifics and we'll take a look!