A spokesman for the mobile network, which has more than 22 million customers, said it was investigating the failure and aimed to fix it as soon as possible. The firm did not identify the cause but it affects outgoing messages, with customers receiving "network out of order" messages from O2 when they press send. The firm believes it affects a percentage of pay monthly, but not pay-as-you-go, customers. O2 could not say how many customers had been affected but insisted it had received relatively few complaints. A spokesman said the fault struck on Tuesday, but was "utterly confident it's not a 'significant problem' kind of number [of customers]".