Have you accidentally placed contact phone numbers in different places in your notebook? OneNote is searchable by itself.
If you know who you’re looking for, just click in the search box (upper right corner) and type the name. To locate all the phone numbers, go through your notebook and for each phone number you find, click the Phone Number tag from the Tags gallery, or use Ctrl+9.
Now, click Find Tags, and you’ll see all the information pop up in a task pane on the right.