Hi @JBaunach,
If you can’t find the Mobile phone number in the left contact record sidebar, you can add this property by following these steps: Customize default record sidebars
Navigate to Settings > Objects > Contacts > Record cusotmization > Default properties sidebar > Edit, then search for Mobile phone number and add it to the About this contact section on the right. Hit the Save button and you should be set.
Whenever you’re trying to find a property, you can also scroll down said sidebar on the left until you see the View all properties button. This is where you should find all information about your contact.
Let me know if you have any follow-up questions!