How to make incoming SIP calls work on Android 4.x?

I have a SIP account which I entered into the "Internet Call Settings" of the native Android phone app. I am able to place outgoing SIP calls, but no luck with the incoming ones. I am new to the SIP subject, so I was wondering what I am missing and a description how incoming SIP works would be a plus. **Info:** - Phone: Nexus 4 - OS: Android 4.3 - SIP Provider: DID Logic (sip.didlogic.net) - Google Voice account: available I am on T-Mobile $30 unlimited data and text plan in case it makes any difference, but it still does not work when I'm at home on Wi-Fi. How does the inbound SIP work? Does the phone or Wi-Fi need to constantly listen for a signal on port 5060, or does the phone do a simple keep-alive connection with the SIP server in the background?