You have to FORCE the issue with carriers!
1) NI-1 is not AT&T custom and will not run, the Nortel protocol in their LT_DEF is U-449 on a DMS-100/250.
2) AT&T custom is U-459 in the DMS-100/250 LT_DEF--INSIST ON IT!!!
3) NTNAPRI is a Nortel protocol for Nortel switches, it will work on Magix switches if set for DMS-100 and NTNAPRI, but be prepared for Hiccups, it is their NI-2 and it will work for a while. When it screws up, you can usually reboot the switch and it will clearup for a while. Something to do with the maintenance messaging in the "D" channel, if I remember correctly. This was a compromise developed by Lucent/Avaya because there are so many DMS's out there. You can buy 2 DMS's for the price of a 5ESS.
4)God help you if you ever get tangled up with an old Verizon DEX-5, just shoot yourself!
This comes from much PRI turn-up experience on Definity and Legend/Magix, plus the help of a friend who is an ISDN Guru at the SBC ISDN test center.