The defacto standard for terminal servers for decades in the carrier class
(and below) was the Livingston Portmaster series. We used to have racks and
racks of PM2E's with a mix of isdn and pots cards.
If you want a terminal server, get the one designed by a company that
specialized in nothing but terminal servers and was the top-end cadillac of
the market. Get a PM2E. Ebay has lots of them cheap.
Jay West