Interesting Bell South acquired AT&T! Don't know if an AT&T ISDN phone would work on a DMS. There are some limitations with the particular 5ESS I work on, such as no name display. I don't recall if the AT&T Definity and Prologic systems had a name option with the extension.

An option I never heard of with the PBXs is multipointing. Two ISDN sets can use the same LEN, or port. The SPID in the phone, 01(7 digits)0 designates which extension it is. Makes sense with BRI being 2B+D. A SPID isn't necessary with one phone on a LEN. I haven't installed one, but the frame tech says the 2 pairs from the LEN are just paralleled. Practical initially, but with moves/changes and one of the phones moving to a different building or even just a different floor will mean the multipoint will need to become separated.