Mobile Cap plans Please explain?

[Dansta]

Alright looks like i might be going on a cap plan, as it works out i spend that amount on pre-paid any way but i'll be honest.

i have NFI how they heck they work. i get i pay $49 (when bundled with the home phone $39) amonth and i get $400 'credit' but, this is the part that makes me go "eh?" is do i still pay for each sms/phone call? or is it i pay $39 a month and after that i pay more? can you tell i'm not the brightest spark in the shed?
Oh and how do you find out (if you can) if you're getting near to your cap?

Thank-you

[Ahurani]

I have an iPhone app that tells me how much I've used hehe thankgod