There is a reason for the difference. In the USA, ISDN modems came with the NT1 interface built-in and were expected to connect to the 2-wire "U" interface provided by the phone company off the street. The rest of the world's phone companies tended to view the NT1 interface as service provided equipment and installed one on-site to users, providing a 4-wire "S/T" interface to end-user equipment.
There are also differences in the actual on-the-wire protocol. Europe had a unified "EuroISDN" signaling system, while the US had multiple depending on the type of central office you were connected to. The WE/Lucent 5ESS and Nortel DMS-100 were the most common standards.