Can't contact customer service from the U.S.

Hello. I was a student in Den Haag but graduated in July 2022 and have been back in the U.S. since August 2022. Before I left I canceled my account with KPN. I canceled with the municipality and have documentation from KPN that I canceled my account. However, I continue to receive a bill for over 50 euros with no explanation about what the charge is for. I really liked the KPN service while I lived in Den Haag but receiving threatening emails about charges that shouldn’t exist is becoming tiresome.

I can’t call the customer service number listed on their site (+31612001200). I called my carrier and they tried too saying it doesn’t have enough digits and isn’t a real number. 

Can someone please help me?