A user interface screen includes names stores in a mobile unit device's address book. In order for a user to send an instant message, the user selects a create message display screen, enters a text message using the mobile unit device's keypad, and then selects a recipient (an instant message name) from the address book user interface screen. The instant message routing server checks availability of recipients (instant message names in the stored instant message name list) and sends the results back to the mobile unit device. In a display area, recipients are indicated as signed on the instant message system, or as being signed off the instant message system.