3Com 3C10402B Administrator's Manual page 193

3 From the CO Switch Protocol drop-down list, select the appropriate
protocol.
4 Click OK.
If your NBX system connects to the PBX system by means of a T1 line, you
must configure the T1 Group settings:
1 On the NBX system, click PSTN Gateway Configuration > T1 Groups.
2 Click the appropriate group name.
3 Modify the fields appropriately. See the online Help for more information
about the fields.
4 Click OK.
Obtaining a Dial Tone from a PBX System 193