When i answer a incoming call, my K800i crashes.
If i try to dial a number, then they answer, my K800i crashes.

Receiving and sending SMS' works, but why does my K800i crash when calling?

Anyhow, how do i solve this?
My firmware is R1CB001, branded T-Mobile, language set thingy T_MOB_INT_A .
Should i send it to Sony Ericsson, or is there a way to solve this without sending it?

You could try to flash it but you'll void your warranty. Usually they don't notice or care but you should hand it back to SE if you want to be on the safe side.

I don't think i have warranty anymore, my K800i was made '06 week 27, and i got it it in week 40, so i don't think i have warranty anymore.

Should i still send it to SE, or should i try risking flashing it?

There's nothing you could lose if it's not under warranty anymore
Just make sure you understand the tutorials and do it correctly. I'd suggest you make a GDFS backup before you start.

Just made a GDFS backup.
Thanks for helping me.

By the way justme, do you know anything of flashing EROM + dead phones?
http://forums.se-nse.net/index.php?showtopic=36193

Can't really say what went wrong on that one. Did you follow the steps of mikhan's tutorial 1by1?
The service cable could help, but I've got no idea where to get them in NL.
I've got all the tools here that could revive the phone if it's not a serious hardware fault, but sending it back from germany would already be about 10-15€ (insurance included, there are much cheaper offers w/o insurance), don't know about the shipping costs from NL to DE.

Fixed it by buying a Cruiser UC10 cable, phone works yay!