ml> personally, i don't see any problem/difference between ISDN and
 ml> POTS other than one being digital and the other being analogue...
 ml> i've never had any problem picking up my POTS telephone and calling
 ml> someone with an ISDN phone and talking to them... yes, i know there
 ml> are several variations of ISDN that are incompatible with each
 ml> other but that is taken care of with the current ISDN flags, right?

There is at least one form of incompatibility between a POTS only system
and an ISDN only system that isn't quite straightforward from current
nodelist practice.

That's when the ISDN system uses equipment that doesn't support any of the
traditional telephony modem communication standards (but only digital ISDN
standards).

This is probably one such example from nodelist 2002:186:

,540,Minas_Morgul_BBS,Goettingen,Marius_Faust,49-551-7988024,300,CM,XA,LO,X75

Yes, it's possible to deduce that this system probably isn't reachable by a
conventional modem for dial-up POTS lines only. The announced maximum data
speed is 300 bps and there is no telephony modem communications standard
(such as V.32bis, V.34 or V.90) announced.

But I would be surprised if there is no mailer out there which at least may
take some manual reconfiguration not to attempt calling that particular
system when presented with a piece of non-routable mail destined there. Not
to speak of what it would take to redirect that mail to the host or hub of
such a destination.

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