I have seen this mentioned in several places, but never tried it until now.

DDTM = Data Dedicated Transmission Mode

Basically, it means if you are on your Instinct in a data session, i.e. web browsing, SprintTV, radio, NFL Live, etc, and DDTM is on, all incoming calls go directly to voice mail. If you turn this off, the incoming call will interrupt your data session.

The best set of instructions I have found is on the Instinct-Samsung.com forum here

Quoting the first post in that thread:

cliff notes version:
I tried both ways with DDTM and can verify that this works.

You need your MSL to do this mod.
Go to keypad and put in ##DEBUGMENU#
go to toggle DDTM and switch it to “OFF”

that’s it. Now phone calls can come through while you’re watching TV or listening to the radio, etc…
WooHoo!! Thanks AutoPrime and Aristo!!!

NOTE: This is a hack – use at your own risk, don’t complain to me if this bricks your phone.