Don't post your mobile number in a public forum!

You need to send the termination letter to o2, not CHECK24.

Hi @Shelat, 

welcome to our English Community. I regret that you like to terminate your contract. 

@bs0 is totally right. Please send your cancellation letter to the following address: 

Telefónica Germany GmbH & Co. OHG

Kundenbetreuung

90345 Nürnberg

You will then receive a confirmation within 2-3 weeks. 

If you have any further questions, just feel free to contact us here in our English Community. 

Loving greetings,

Tobias