IMHO there are many advantages to converting to ISDN,
If we speak of BRI (basic rate interface), it is still normally delivered over a single pair (U interface) , the restriction is that you need to be closer to the CO (exchange) than an analog(ue) line limit.
You will normally get two 64k clear channels so two concurrent voice calls and two separate phone numbers.
Either B (bearer) channel can be used for voice or data, plus you can bond 2B+D to 144k for pure data, depending on what TA (terminal adapter) you choose and who your Provider is.
These channels are digitally (the D in ISDN) so all the old problems of tx/rx level and fxo tuning are pretty well gone as there is no “analog hybrid bridge” to worry about, the TA will take care of providing that “hybrid” back to analog if you choose the right TA (the ones with an RJ14, nicely pre/de-mphasized to ALAW or ULAW, on them for connecting your old “princess” phones to ) (very useful for power outages)
The 16k D (data) channel carries the signaling “out of band” so calls are “built up” and “torn down” almost instantaneously, CLID is also delivered over the D channel, so normally it just works without muss or fus and the caller doesn’t need to wait two rings before your phone rings, or a few minutes to hang up until the co sends out it’s beep beep beep “you forgot to hang up!!” signal.
If money is of little importance then ISDN PRI (Primare Rate Interface) is definetly the way to go, 30 (23 in NANP land) concurrent 64k channels for voice or data, as many “phone numbers” as you care to subscribe to, plus all the previously mentioned advantages.
In either case the TA needs power to work, some vendors supply BRI over a “wet pair” that supplies that power. Germany where ISDN is the norm has the better manufacturers of TA’s due to their experience, I would keep away from cheaper and more oriental adapters/clones as they always need a computer and power and have been known to not always work so well
Emergency services - 911, is 999 and or 112 in the UK, so I infer that cdsJerryw is from NANP (North American Numbering Plan) land where the local loop is usually a lot longer than in Europe, (The length limitation is similar to that of DSL and it’s varients, but the two data services are mutually exclusive of each other, be aware of that!) and where BRI’s are almost unheard of.
So although the technology came originally from AT&T (Bellcore Labs.) it just never was practically deployed in the U.S. except perhaps for the entertainment industry, I have many clients using one or more BRI’s in their Hollywood Hills home studios, going live on air all the time, there are no delays as happens when the cheaper Internet based solutions are tried.
Basically the quality of service and sound is very much better than any analog line can be.