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Think of the configuration as the unit's "role". One end of a PRI will always be the network (NT) and the other will be the user (TE).
The carrier side is the network and the PBX is the user.
If your 900e interface is connected to a telco PRI, that interface will be role user, because it's connected to a network.
If your 900e interface is providing service to a PBX, it will be role network, because it's connected to a user device.
If you're bookending two 900e's back-to-back via PRI such as a tie-line scenario, then set one to network and the other to user. It really doesn't matter which is which, but by convention the hub is usually the network and the spoke is the user.
By default, the device is set to role network, as it's most often used to convert SIP trunks to PRI facing a customer PBX. It can be changed to role user under the PRI interface with the interface-level command role user. You can have different interfaces with different roles on the same device. Under role network, there is an option to periodically send b-channel restarts. This is a means to ensure that all of the B-channels on the PRI are in the same state (busy or idle). B-channel restarts are only configurable in network role and are disabled by default.
If you haven't changed the PRI interface setting, then your 900e will be set to role network, so you'll need to set the T-Berd to user for the D-channel to sync up.