Call forwarding/ external 'extension'

I’ve just set up asterisk… and am astonished how quick it was to set up… great, thanks to all devs!!! (Used asterisk@home,though)
Now, what im trying to do is setting up an ‘extension’, which is my mobile. So, for examle, if I call 205, it calls my mobile. It would also be very easy to do call forwarding for after-hours etc. I tried it with a custom extension, putting my mobile number in the dial field, but I get the error that the number is not “in the speed dial”.
Any thoughts?

Thanks a lot in advance,