my mobile phone was returned by DHL

you can do nothing. 

Plesse cancle the Unlimited Smart via E-Mail to widerruf@cc.o2online.de 

Hello ​@Ni k,

welcome to our o2 community 🙂

That is unfortunate, that your order did not arrive at your current address. If it is returned to us, your order will be cancelled automatically and you will receive confirmation.

Could you already cancel the mobile contract as well via our e-mail given by Bollermann?

Best Regards,

Gerrit

have already sent an email regarding the cancellation of my order to widerruf@cc.o2online.de. However, today I received a message stating that my shipment with order number xxxxxxxxxxxx was delivered.

I would like to clarify that I am not concerned about the delivery time of the device. My concern is to avoid any misunderstanding, as I am worried that the contract might start without me having actually received or accepted the device, which could lead to complications.

Hello ​@Nick1107 

It's good that you canceled the contract.
Have you already received confirmation of this?
We cannot access your customer data here to check this.
If you have any further questions, please contact our English hotline.

Best regards
Marika