Yeah this is like asking to please have rotary dial service only..or trying to find a new black and white TV. I was actually "paid" by our rural telco to discontinue my ISDN because I was the only one with it in my part of the state, and they had to pay a fellow full time money to sit around in case I had a problem. It ended up costing them less to pay me off then maintain one person's full time position.
I am now using Source-Connect on a PC and for the price (waaaaaay cheaper than a Zephyr) it sounds better, and if setup right is just as reliable. PLUS there are ISDN bridging services so you can still do real-time actual ISDN sessions with clients (I do commercial VoiceOver) by just giving the bridging service SPID numbers and connecting with Source Connect to the bridger. About 40 bucks per half hour..and that..compared to a monthly rate of 172.00 for each month is a better deal..