Payment from abroad

I worked in Germany as an aupair from August 2017 to July 2018. When I left to return to the USA, I shut down all my accounts (including my phone plan) but last week I received a notice saying that I haven't payed my O2 bill and I have 3 days to do so.

I no longer have a bank account in Germany so I am going to have to use an online payment system like Xoom or Transferwise in order to pay. All of the online payment systems want me to specify an email address to notify the business that I'm sending the money to that the payment is on its way. Does anyone know what the appropriate email would be for this? I cannot find ANY email addresses for O2.de anywhere, but without one I can't pay!