I have not installed one. From the book:
If a 16 channel rdtu is installed in expansion cabinet number 2 the allowable slots are 21, 23, or 25. No slot must be vacant, however it will not be possible to turn on channels 17-24 later to add more T1 lines unless the next slot after the rdtu card is left vacant. If the T1 is in 23 you would only need to leave 24 vacant
to take full advantage of the card. Once again I have not installed a rdtu.
JBW