T-Mobile, I'm guessing, as I have the same plan. Look at http://prepaid-phones.t-mobile.com/prepaid-plans. It's near the bottom, under "Pay by the day" even though it's a monthly plan.
$30/mo. for 100 minutes, unlimited text, 5 GB LTE data, throttled to 2G but unlimited with no overages after reaching 5 GB. And their LTE is fast as hell, I have pulled 30/10 on it which is faster than my Qwest/CenturyLink DSL (at the fastest speed they'll sell me).