To add a little more details, here are the instructions that would need to be automated in a mortscript:
dial ##33284#
Select (B) Field Test Menu
Select (G) DDTM
Default is "ON", change it to "OFF"
Hit BACK softkey then BACK again and EXIT.
DDTM resets each time the phone is turned off so you'll need to do it each time you power the phone up. Just add a contact to dial it.