Techniques are described for forwarding an instant message addressed to an intended instant messaging recipient to the recipient's mobile telephone when the recipient's mobile telephone number is known to the instant message sender. In one example, an instant message is forwarded to the intended recipient's mobile telephone number when the instant message sender's contact information for the intended recipient includes an instant message address (e.g., a screen name or other type of instant message identifier) and a mobile telephone number, without dependence upon the intended recipient's presence on the instant message (IM) system or through actual login or registration with an IM forwarding service. In a more particular example, an address book of the instant message sender includes an entry for the intended recipient where the entry includes an instant message address and a mobile telephone number of the intended recipient.