I don't know, it's looking awfully like a Spark network issue...
Given every device so far (that I know of) exhibiting issues is using the Spark network in some fashion, regardless of where the device is sourced. Again, is there anyone who has these issues and isn't using Spark at all? Maybe there is an issue with the Spark profile that is being distributed OTA?
@ShinyChrome Does look like a conflict with the SparkNZ network but never rule out the handset
Logs off the Spark network and logs off the handset will prove where the issue is and I bet it's a simple fix