Voicemail is associated with your One+ mobile subscription.
If you cannot answer your phone, Voicemail will receive the message and notify you by text message. In addition, Voicemail is built into the Assist app, where you can see a visual overview of the messages waiting for you. This gives you an quick overview and you can jump directly to the playback of a message by tapping it.
Once you have switched to One+, you can record your welcome message, listen to recorded messages etc. by calling your voicemail number.
Your voicemail has phone number +45 20171717.
Read more about your voicemail on tdc.dk on the page Kom i gang med din voicemail (in Danish).