A proxy for SMS-based wireless instant messaging (IM) is located on a network server. The proxy provides an on-line IM presence for a mobile device user and facilitates multiple cross-platform IM services without requiring protocol specific functionality locally installed on the mobile device. According to an embodiment, the proxy is configured with information about the user such as userID and login information. In operation, the proxy may log into a plurality of IM systems and provide an indication of the user's presence. Upon receiving IM messages directed to the user, the proxy may 1) automatically reply; 2) forward a converted message to the user's current presence; 3) forward a converted message to the user's on-line message repository; and/or 4) store the message for future delivery. The proxy may also provide presence information of contacts for the mobile device user. Upon receiving SMS messages from the user, the proxy may 1) determine the appropriate IM protocol; 2) log-in the appropriate IM system; 3) send the message to the intended recipient.