Looks like Spark is experiencing major 4G/LTE outages.
https://i.stuff.co.nz/business/128635576/spark-customers-struggling-to-make-or-receive-mobile-calls
I tried to send pxt yesterday and failed. Now I know why. It did not drop back to 3G for that service. Looks like that is part of the fault too.
NZHerald says 20% of VoLTE calls are failing. Not much detail in there.
https://www.nzherald.co.nz/nz/sparks-volte-connection-issue-causes-20-per-cent-of-calls-to-fail/JPEM4KACBM2AUGNFRZF7MX7ZPA/