Redo your Combo box. Select the Name and the Phone# ... in that order (for this example). When setting the size of the combo box, make the size for the phone# 0 (size it using the mouse pointer).
In the On Lost Focus or On Change Event for the combo box, enter the following:
Me!MyPhone#Field = Me!MyCombobox.Column(1)
The way this works is: Your combo box contains two fields ... Name and Phone#... but Phone# is hidden. Each column within the combo box is numbered, starting with 0. So, when you select a name, you're selecting Column(0) of the combo box. Your code would then enter what is in Column(1) of the combo box into your phone# field on the form.
Me! ... is simply referring to the active form
MyPhone#Field... is the name given to the field you want the phone# displayed in.
MyComboBox.Column(1)... points to the hidden phone# related to the username selected in the combo box.