Hello hanspumpel
Sure, you can use a custom / child theme, see for example the https://www.rosariosis.org/wpadmin-theme-icon-sets/
Then, in your stylesheet.css file, add
.public-pages-staff .phone-link,
.public-pages-staff .phone-link ~ .legend-gray {
display: none;
}
That being said, this solution only hides the phone number. It is still available in the HTML source code.
But if you want to edit PHP code (and thus maintain your customization when upgrading the plugin), you can search for 'Phone Number'.
One thing to remember when trying to spot where in the code is something displayed on the screen: try to search for the text displayed, like the field label "Phone Number": 'Phone Number' with single quotes.
Of course, you will need a code editor with the capacity to search on the whole RosarioSIS / add-on folder.