-1 There's no hard limit for voice, completely honor system.
Data is "unlimited" 5gb.
They actually used to have a whole system for monitoring (and eventually limiting or just booting you off completely) cellular usage, but it looks like they've done away with that.
And just to be clear, the $10 plan that was being discussed is not 'unlimited' 5GB data - it doesn't include any cellular data at all. The $25 (3G) and $40 (4G) plans, which do include cellular data, are not explained very well. They both provide unlimited cellular data, they just throttle you down to slower speeds past 5GB (per month) - which is what a lot of the other carriers do as well. And even then, they won't throttle you at all the first month you go over, and that 'free pass' refreshes every 6 months that you stay under the 5GB.