Truth be told though, it's all on their web site, but is written in an unclear manner. Their plans are generally confusing if you aren't used to such strict top off rules. It sounds like you may want to choose another plan. Of course any free plan will generally have several stipulations, which is clearly what you have run into. See link:
If you are using more minutes, you may want to compare with simply switch to a prepaid provider. AFAIK Assurance uses Virgin Mobile service anyway (=Sprint network), which has pretty good rates overall. YMMV.
I am a disabled person who reeived an assurance wireless cell phone. as much as i appreicated this, there rules are strange. you receive 500 free minutes a month and then they tell you can purchurse more minutes. I purchaed 250 more minutes and rain out and put more money on the phone and was told i could only add minutes once a month. this is not mentioned on their aids on TV or anywhere else. I have important calls to make. I am told to pay 10 cents a minute. They are dealing with poor people. I was lucky enough to add $10 for more minutes. They will only accept $20.00
be careful...you get what you pay for.!!!