Dear old Ma has been migrated from copper to wireless landline. 

Reading documentation she had at hand, it costs $50/m, with mobile & 'toll calls' incurring a p/minute charge. 

Also in there was a statement "you will not change the sim card in the modem"

Out of pure curiosity, what would happen if I were to pop a Skinny sim with a $17 prepay plan in?

She has a St John medical alarm too if that will have any bearing 

She has tried a mobile phone, too confusing - fair enough.

Thanks for your time