@Anonymous If your bill is in arrears, and you haven't sorted that out, I'm not sure what you expected to happen? You say you have an agreement in place, but I'm sure there's nothing in that agreement that says you can be in arrears. I doubt you'll find any phone provider who will let you carry on using their service if you're not paying your bills.
The problem is you are in arrears with your bill payments. Whatever the reason you haven't paid what you owe. That being the case, O2 are well within their rights to bar your phone.
Customer services are certainly closed at the time you posted, so yes you will have to wait till 8am to contact them
The bar won't be lifted until you have settled your bill.
If you set up a direct debit (which is part of the terms and conditions of your contract) you could avoid this inconvenience in future.
Veritas Numquam Perit
Any verbal agreement you had with customer service would be nullified by the indiscriminate automated system which no one can override. I doubt even calling them in the morning would have the desired effect of reconnection unless a payment is made. The same system will quickly place the restriction back on the phone should an adviser remove it.
Reading your post @Anonymous made me laugh. You are in arrears due to an unpaid bill and not for the first time as you mention 'constantly having bars removed'.
No mobile operator will allow bills to go unpaid and continue to provide service. O2 is not unique in this respect and they are well within their rights to do what they've done; read the T&Cs.
Everyone who has replied on this thread, including me, pays their bills by Direct Debit thus avoiding late payments. Not being happy with the charges you incurred whilst abroad is no excuse not to pay your bill. That's what customer services is for to sort out queries, and arrange credits if a mistake has been made. And no they aren't open 24/7 but certainly enough hours for you to have contacted them and sorted out any queries and a payment before a bar was placed on your phone.