ADTRAN TA 900 Quick Configuration Manual page 6

The last step is to create a trunk group for the RBS trunk. To start this process,
click on 'Trunk Groups' under the 'Voice' tab.
Give the Trunk Group an appropriate name using the 'Group Name' field and
then click 'Add'.
You can give the Trunk Group a description to help keep track of the
configuration.
To add the RBS trunk to the trunk group, click on the 'Add Members' button.
A new window will open with the available trunk accounts. Check the box under
the 'Add?' column for the appropriate trunk account and then click on the 'Add
Selected Trunks' button.