Hello HM and LL
I would interpret each version as follows:
1. A prepay mobile – a mobile where you pay for your usage in advance.
2. A prepaid mobile – a mobile where you've paid for your usage.
I'd be more comfortable with #1, as #2 implies that you've already paid for all your future usage.
('Prepaid envelope', for example, is a slightly different case, as you only use the envelope once.)
MrP