It's a very individual process for each customer, to be honest.
Using different factors such as the customers credit score and previous calling habits it can make a decision. generally there will be a text sent to make sure it is authorised usage or to warn that you've use £x over, but if it gets higher than the limit the system has allowed it will just bar it and request a partial payment before it can be unbarred, the request will either be made by text or email.
As I say it's completely individual to the customer as to what to the cut off point is.
5.10 We can’t set usage limits on your account. We’ll try to monitor usage of
the Service through your account to control our credit risk and your exposure
to fraudulent usage or unintended Charges caused by your usage, but we can’t
guarantee this and O2 is not responsible or liable for any such fraudulent usage.
If we see usage on your account that causes us concern, we might restrict use of
the Service on your SIM Card and/or bar your SIM Card. You’ll need to contact us
before you can use any of the chargeable aspects of the Service again. You may
also have to make an interim payment before the Service can be restored. You
may be able to track your own usage on our Website.
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