That’s not how I understand it, & not what my friend who has used it told me.
Look at the prices
here
You pay $40 for the initial sim which comes with 10 credits.
You can then add however many credits you wish at the prices shown in the link. Ongoing cost is not $40 for 10 units
Annual subscription costs 39 credits (US$45.50)
They give you 3 months ‘grace’ past the annual renewal date to renew without needing to pay a new joining fee (i.e. a new sim or re-activate old one)
So essentially you can have the service available for US45.50 per 15 months & only ever buy extra air time if & when you need it from anywhere in the country using a free sat phone call. Or you can buy air time so it’s there to use without first needing to make a free top up call, & if you don’t use, it rolls over to the following year when you renew your subscription (also by free call).