The Adtran Atlas 550 series of IAD devices must be one of the most reliable hardware networking devices available on the market today. this series has been available and is in so mnay networks it's hard to find another device in this category as an equal. I have one of these units and I've installed them for customers to. They are easy to program and configure ( some good foundational knowledge assists greatly). They are modular and support a large number of technologies.
My Adtran Atlas 550 has two v.35 interfaces two t1/PRI modules and 4 port BRI and a another network module. I've used it for simulating frame-relay networks and also setting of equipment for customers before I going on site to the customer's office. Also used it for testing equipment. and have installed several for customers as a channel bank and a multi-point CSU/DSU.
They are reliable and because they're modular purchases for only what's needed but having room for expansion is a no brainer. I recommend the Adtran Atlas 550 to anyone. Cisco routing and switching is reliable too but they are so much more pricey than Adtran.Adtran's support is very good too and that's very important when you have a network down emergency or even during installation. A smooth installation is very important too for customer perception and confidence building. It's good to have good network techincal support that can be counted on.