Unfortunately there are many dialects of ISDN. This was common-place in the old national PSTN networks even before ISDN, so it seemed "normal" at the time.
International / ITU-T (formerly known as CCITT)¶
Europe / ETSI¶
Etsi specified what was later known as "Euro-ISDN".
In Germany, there was a national ISDN dialect called 1TR6, which was later obsoleted by Euro-ISDN. See German_FTZ_ISDN_Specifications.