After timeout is over a SIP account does not register again automaticaly, manualy can be unregistered and registered again. Changing timeout seconds does not make any difference, re-registering does not happen.
Is there is a way ekiga can be forced to re-registered itself automaticaly ? 
I’m not SIP expert, but IMHO if it doesn’t it’s a bug. Report it at http://bugzilla.gnome.org/enter_bug.cgi?product=ekiga