The iPhone keeps a list of people you can call in the Contacts app, where a great deal of information about each person can be stored. Eliminating all of this information so that only the phone number is seen in the Contacts app requires editing out all of the information other than the phone number. The editing can be done on the iPhone and takes only a few minutes. None of the other contacts in the Contacts app are affected, and you can restore all of the contact information at a later time should you wish to.
Wake the iPhone by clicking the "Sleep/Wake" button at the upper right corner. Swipe the slider to the right to go from the "splash" screen to the "Home" screen.
Download an iPhone management program to the desktop of the computer, like the free iTunes program that works on both Macs and PCs (see the link in Resources). Double-click on the icon of the iPhone management program when it has fully downloaded. Follow the menu prompts to install the iPhone management program to the computer's hard drive. Restart the PC after the installation is completed.
Connect the iPhone connector of the iPhone connection cable to the iPhone. Connect the other end of the iPhone connection cable to a USB port on the computer. Double-click on the icon of the iPhone management program to launch it.
Use the mouse to click once on the icon of the iPhone that appears on the left of the iPhone management program's main screen. Right-click on the iPhone icon and select "Backup" from the pop-up file. Wait as the iPhone management program backs up the contact information among other information from the iPhone to the computer's hard drive. Use this backup with the iPhone management program to restore all of the contact information that was deleted from a contact in the "Contacts" app if desired at a later time.
Click on the "Info" tab at the top of the iPhone management program. Uncheck all of the check boxes on the iPhone management program's main screen so that a sync with the iPhone will not alter the contact information saved to the backup file that was just made. Quit the iPhone management program. Disconnect the iPhone connection cable from the iPhone and from the USB port on the computer.
Tap the Contacts app. Tap on the name of the person whose phone number you want to only show. Tap "Edit" in the upper right corner of the screen that appears.
Press the red "-" button next to every column except for the column with the person's phone number. Press the red "Delete" button that appears at the right side of each column to delete the information in the column. Tap "Done" at the upper right corner.
Do a second backup of the iPhone using the iPhone management program, so that the first backup that has all of the contact information intact will not be accidentally overwritten by further backups. You can also search for the iPhone backup by entering the name of the iPhone in the computer's "Search" field, and then copy the backup file to another file or a second hard drive for safety.
Contact information that has not been backed up and was deleted must be re-entered manually into the iPhone if you want to have it again.