First login as admin.
Click on Rates on the button bar on the left.
Now click on the tab Routing Tables at the top of the screen.
Click on the + to add a new record, filling in the following:
RatePrefix : {leave it blank}
Line : 050 {Enter the line that has been configured as a tie
line on the pabx}
UsePrefix : INTERNAL {This will make all calls internal calls on this
line}
Description: YourOwnDescHere {You can use this for your own Description}
Click on the Tick at the top of the screen to save the line.
This procedure will need to be reproduced for each line that needs
to have no cost.
Tie-line could be a diginet line or any type of line that connects two PBXs.