Well, I tried a lot of things to get this sipcall.ch account working and well now it works.
First a few words about my setup: I use a vmware trixbox, but i think it could also work with a simple asterisk installation.
;for outgoing calls
username=4132XXXXXXX (your number)
secret= <your password>
fromuser=004132XXXXXXX (your number)
; For incomming calls
So as you migth see the magic part about my config is the duble zero before my sipcall.ch account number in the fromuser field.
Hope this helps for me it works very well. And by the way i tried this useragent=anything trick and it had absoluetly no effect on my system.