Therefore, O2 get the money paid directly into their account, with no "admin" required.
Actually a direct bank transfer requires admin. More admin than a DD.
Reason? On your bill you'll find your customer number, invoice number and maybe a few other bits and pieces. You've got to copy these manually into your online banking program. If you get these numbers right, the O2 computer can automatically match the transaction with the invoice and thus mark the invoice "paid", with no admin required. However, every now and then somebody doesn't understand the purpose of these numbers, or makes a typo. And then it requires a human being to match the transfer to the proper invoice. I have no statistics to hand, but my guess is between 1 and 5% of these transactions requires some form of human intervention. Which is expensive.
Whereas with DD, the invoice, the transaction and their match is all done by computers. Who typically don't make mistakes like that.
Furthermore, specifically for telco companies: Their invoicing/admin system is highly sophisticated and fully automated and they like it that way. After all, these companies have to make a *profit* on each 2 cent phone call. You can't do that with a billing system that costs more than a euro or so in admin fees per invoice.
In contrast, I work for a large IT firm and if you take the total cost of the accounts receivable department and divide it by the number of invoices they handle, you get a ballpark figure of 100-150 euros per invoice.
Grumpy old git hat firmly on, I shall be on the dog and bone this morning. I'm quite sure it won't do any good, but ....
I know the feeling. But my guess is that for the same reasons I described above, you'll end up talking to a (cheap) call center monkey in India who will very politely and very patiently explain to you that that's their policy and no, they're not going to deviate from that for you. And yes, you can take your business elsewhere but you'll find that elsewhere the same policy will apply.