in order to not been disconected your call, you have set the registration expiry to less than 600 second(my understanding), some one said less than 113 second, and this is from sipsorcery. I used 180 second and it works.
here is the link http://www.mysipswitch.com/forum/viewtopic.php?t=1770&sid=1c48e72a129119de7886f4d8bd09bcf2