How does android.net.sip bypass firewalls?

I'm planning to implement a simple VOIP feature, using the new android.net.sip in Android 2.3, as an extra feature of an existing Android application. Earlier, i.e. before 2.3, I tried to do a naive solution but I could not connect the other mobile phone because of the carrier network operator's firewalls!

So, I wonder, how does this new SIP package in Gingerbread bypass those开发者_Python百科 firewalls, allowing mobile phones to connect directly to each other? Or will there be problems anyway? (I scanned Googles Android documentation but could not find any information on this topic.)

Thanks in advance!

/Steve

You're making an unwarranted assumption that there's a (hidden) mechanism for bypassing/traversing firewalls. Maybe there is, but quite possibly there isn't. The overall SIP documentation looks thin; this feature may well not be ready for general use, or you may well need to implement such things outside of it (STUN, TURN, UPnP, etc).