I have a single fixed IP, associated with my login name. If I connect with either analogue, iSDN or ADSL, that connect gets assigned that IP. I run NAT behind the IP. At one point I was running 12 non-routable addresses behind it. I've not tried seeing what happens when I try more than one connect at once: I suspect it will be rejected. This is not a business account, it's a standard private account with XS4ALL.

The Zyxel ADSL router (a P314) had fallback from ADSL in it as far back as 2003 when I bought it, and it was already a year old model then. Likewise, my more recent ASUS W500g wireless ADSL also has it, with extra features. It just means I can run one ADSL and one fallback iSDN router on my LAN with one fallback path through the iSDN.

I pay an extra E5 per month for the fixed IP, but otherwise no other charges above the ADSL over iSDN account fee. The account fee is E32 (ADSL) and the equipment hire fee to KPN is E35 (iSDN).