I have a DECT 6.0 cordless and it is on do not disturb mode and i dont know how to fix it

Question is kind of Vague, so I'm assuming you are trying to turn off the Do Not Disturb (DND).
In the standby mode, press the do not disturb button on the base. The DND indicator will light and DO NOT DISTURB shows in handset display. To cancel, press the do not disturb button again.
Hope that helps.

What model Uniden DECT 6.0 phone?

Hold down the # key to get rid of the Do Not Disturb on a Uniden Premium DECT

That depends on the model. Some models have a do-not-disturb button, others require you to change the ringer volume. Since you didn't specify the model of your phone, select it from the list beginning at
http://www.uniden.com/owners-manuals/dect-phones/scat/dectphones_mnl/
to get the manual.

Since you declined to specify the model of your Uniden DECT 6.0 system I can only suggest you select it from the list beginning here to download a copy of the manual.

Apparently you have the "Do Not Disturb" turned on. I wish I could tell you how to turn it off, but I can't do that without knowing the model of your Uniden DECT 6.0 phone.

Just press the button. It goes on for Do Not Disturb (solid red light). Press again to turn off.

press "menu", "global setup", "dial mode", choose "pulse".
I dont know why it works, but it does!

http://www.home-electronics.net/ge/pc/IB/28115FE1_IB.pdf

GE phones are distributed by Thomson Electronics...the manuals are available at:
www.hom-electronics.net

According to the instructions, you only need to press the "do not disturb" button again.  The button is on the base (not the phone).