It's been a very long time, but as I recall, a 19 station Intercom uses (for example) Intercom numbers
1,3,4,5,6,7,8,9,0, 20, 21, 22, 23,24,25,26,27,28,29.
2 is the transfer digit in this example. (It could just as easily be 3 or 7 or 9 or any of the other digits, but 2 was standard practice in the US).
If you're adding another 18 stations then you need another "tens" group so for example, you would remove 3 as a single digit extension. Your single digit extensions are now 1,4,5,6,7,8,9,0.
You still have 20,21,22,23,24,25,26,27,28,29.
You now set up 3 as another transfer digit and now have another group of ten extensions 30-39.
Sam
