Belal,
DTX means the transmission of bits stops during silence. As I know, the TCH is always emitted by the tower wether there are calls or not.
Accounting DTX as one of the features that reduces the interference is same saying:
The amount of interference in the air interface depends on the number of bits transmitted (number of calls). Is it??!
If the answer is NO, then DTX has no hand in reducing the interference.