Hi Susanne and welcome to JustAnswer,Thank you for the question and your patience. Should you still need assistance for information purposes only...Initially, bear with us; re-confirm that the model number of the Panasonic is KX-TGD213N.
Appreciate the post back and confirmation.
Who is your phone service provider please?
The KX-TGD21?? series of digital cordless phones do not have a built-in answering system. The next series (which looks like the KX-TGD21??), the KX-TGD22?? are the versions which have a built-in answering system.
I know this isn't what you are hoping to hear, and I'm sorry I don't have better news.
I realize that the above information may perhaps be not what was expected. Unfortunately, it is a design limitation; sorry for having to be the bearer of bad news.
There may be a possible / hopeful workaround. Would this be of interest?
If yes, please contact your phone service provider and have VoiceMail feature enabled. VoiceMail serves the same purpose as that of a built-in answering system only that it is with the phone company's servers rather than in/with the KX-TGD213N phone.