Just looked up my wife's phone bill which was double $30 ($60) and reminded it was a double bill month as their billing periods are 28 days
All well disclosed upfront but I did not think much about it
But 28x12 = 336 days. 29 days short of a full year, I.e. An extra bill cycle
So when compared to a true monthly plan the $30 plan is really $30 x13 / 12 = $32.5 per month.
Are most prepaid sims on a 28 day cycle?
Yes. It's how all phone companies screw over their pre-paid base.