AT&T MERLIN LEGEND Release 2.0 Equipment And Operations Manual page 156

Lines and Trunks
The framing and signaling formats depend on the network and interconnection
devices (CSUs) used. For example, many CSUs only support ZCS line coding.
NOTE:
Digital data up to 64 kbps is possible only in PRI mode.
An ESF-T1 CSU must be used for interbuilding DS1 connections.
3-22 DS1 Facilities