Mystery solved, but as I have unlimited texts and calls on my plan, it still rankled to pay for PXTs on top. I never really use MMS because apps that run over IP data connections let you send much larger and more detailed pictures and videos in a faster and more convenient way.
MMS is really expensive throwback tech: the Spark and 2 Degrees websites say the two telcos also charge 50c per PXT, and $1 per video. Looking at the PXTs on my phone, they were about 160 kilobytes in size; tiny low-resolution files compared with the multi-megabyte images smartphone cameras take.
On Vodafone, that works out as $0.50/0.00016, or $3125 per gigabyte of MMS data.
In comparison, I pay $79 a month for 22GB of IP data on my Vodafone plan, or $3.59 a gigabyte — less if you don't count the unlimited calls and texts that are included. MMS, by the way, runs just fine over TCP/IP.
A wave of nostalgia swept over me as I looked at those figures. I remembered the bad old days when using your mobile phone was playing Russian roulette with bankruptcy. What fun that was.
On a more serious note, don't use MMS if you can help it. The feature is usually turned on by default though, so make sure you go to the settings panel on your smartphone and switch it off.
Otherwise, you might find that group texts which you thought were sent as SMS texts become MMS messages, and charged accordingly.
Ditto emojis, which are even more annoying to send and receive at 50c if the messages are converted to MMS.
Presumably, the high profit margins on MMS mean there's little incentive for telcos to dump the service as they race to the bottom to become dumb pipe carriers, and desperately try to eke out more revenue from their networks while they still can.
Leaving MMS on and stinging people with ridiculously high charges for doing normal 2017 things like sending pictures and videos is bad public relations by telcos that alienates customers; perhaps telcos should think about the bigger picture?