Reading over the reasons why Android isn't "100% supported", this looks likely to end in frustration. (Though I have not tried on an Android, nor do I know if the capabilities of the SIP client have improved.) I know you said you weren't open to 3rd party apps, but I highly recommend Bria for SIP on a mobile phone. Better control over codecs, and more important, the ability to tune settings to ensure the app stays registered. If the native Android client has issues staying registered, this would result in missed inbound calls.