as the title says, my sister needs a contract but cant be trusted with a contract so is there any network which stops your phone from making outgoing calls after you use all your free minuites? am sure orange dont do it.
Same as me! Im on T-mobile u fix and if i use all my free minutes up (750 offpeak) texts or call out of my free call times, i have to go and top up my phone.
As Misanthrope has already said, it's not in the operators interest. From a technical side, many post-paid phone systems aren't real-time calculated, running on the theory that they can do the maths at bill processing time.
Some operators run credit limits - you could try asking them to set the credit limit to the same value (or near) as the tariff rate plus a bolt-on of some sort for SMS (for example). Again this isn't exact - it won't cut you off at the limit but should be close enough. The only thing is that this would probably block incoming as well as outgoing.
o2 put a credit limit on your account, I have a contract in my name for my little cousin (well I say little, aged 16) and I asked them to put a limit of £25 on it, so they bar outgoing calls/messages etc when it reaches this amount