If you have received a Samsung Galaxy S20 with a new contract, then it is very likely that your own phone number is also new.
If this is the case and you don't know your own phone number, you can look it up in the Android operating system as follows
1. Open the settings on the Samsung Galaxy S20
2. Scroll down to "Phone info
3. You should now be able to see your own phone number under "Phone number
If you can't see your own number here, but can see "Unknown" instead, this is due to the sim card of your mobile phone provider.
Not all providers integrate the information for the phone number into their Sim card. This is especially the case with mobile providers such as Premium Sim, Congstar, Klarmobil etc. Providers such as O2, Vodafone and T-Mobile should have this information integrated.
Practical alternative: Save new phone number in Contacts app
You can maintain your own contact in the Contacts App of the Samsung Galaxy S10 in the overview and enter your phone number there.
You will then know how to look up your own phone number on a Samsung Galaxy S20 smartphone.