There are a few reasons about the registration problem, please check:
(1) Make sure your domain, username, password is set correctly, check again.
(2) Some SIP providers asked you to set proxy server same as the domain, this could cause problem in some cases. So if you have "Time out" registration error, please try to clear this same proxy server and try agaiin.
(3) Some SIP providers ask you to use a different auth timeout value, try to use default value or 1800 if you are not sure.
Any third party apps on iPhone can't run in background. So this app can't accept incoming VOIP call when it's not running.
However, this apps does run in the sleep mode. So as long as you are in the WiFi network range, keep this app in sleep mode, it does receive the incoming call. But it's consuming the battery at the same time. Please quit this app if you want to save battery.
With iSip 3.0, iSip supports Apple's push notification service.