Being able to search the roster on the mobile app by phone number means never again having to ask, "New phone?? Who is this?!" when an unknown phone number texts you.
Open the roster from the hamburger menu on the mobile app, and then click "search roster" on the bottom of the screen and paste the phone number you are searching in the search bar. If the number you are searching is connected to a Member's profile, that profile will appear in the list.