Hello i have aproblem with my kx-dt346. the problem is that i can make outgoing calls but when i have an incoming call when i pick up i can hear and talk to the other person but after 3 seconds it hang ups.

the dt 346 is a handset what system and lines do you have

I will try to be as detailed as possible.
I have one line for calling and receiving calls, when the phone rings the number 1 lights up on the soft buttons and when i call someone the number 2 soft button lights up. The problem appeared all of the sudden so i don't know if some button was pressed by accident and messed up with the programming.
As i said when i have an incoming call the phone rings normal as usual, i answer the phone and for 3 seconds i can talk and hear the other person. after the 3 seconds the line cuts off.

Does the light go out or just the call goes.

there is no setting to cause a call to end after 3 seconds.

you will have to get the line tested before the system, if sip you will need to run a wireshark trace to see whats going on.

if you can't do it, get someone in to look at it