Got a bill after Kündigungsschreiben

I had a contract with o2 while I was in Germany.

And I did Kündigungsschreiben before leaving and paid everthing.

Also talked with O2 hotline to be sure I did everything for Kündigungsschreiben.

However, I got an e-mail bill from O2 today.

I already left Germany and don't know what to do

How can I contact to the o2 and check my contract?

 

 

Please help me.