I have been able to reproduce the issue on the Spark V100R001C227B011 and Vodafone V100R001C206B013 firmwares.
This suggests to me either both networks are blocking the traffic OR there is something broken in the base firmware/hardware that these firmwares share.
On a side note, the Vodafone model comes populated with the default Vodafone SIP settings. I put some bogus info into it to see if I'd get a different response. Guess what the logs show... "Registering with VoIP number 07xxxxxxx to Register server sip01.vodafone.co.nz failed: Reason: no answer from remote station received."
Maybe this problem is deeper than we realised.