If you cannot remember the cell phone number or email address of a contact, you can use the search function to get the needed information.
Contacts which do not match the search criteria are hidden in the contact list. The contacts are shown again as soon as you close the search by clicking the "Cancel" icon (x) in the search box.
In addition to the deafult search function of your GMX Contacts you can use a cross-services search by clicking the Search button in the top right corner of the navigation bar. Find more informationen at Cross-service search.