Mobile phone contract help!?

I am thinking of getting a contract mobile.say for example my contract was 200 mins + 500 txts. the i use up all those minutes and txts. will they start charging me? and if so is there anyway that when you run out of txts and minutes you are just unable to send or ring more liek the way a pay and go phone is?
Answer
Yes you will be charge, i think that t-mobile and t-flex contracts only allow you to use the allowance they give you then you have to top-up to use more (t-mobile will give you more info), however with 3 you will get the most minutes/allowance, but i like tmobile better.