A system and method for conducting a client-server application (e.g., instant messaging, VOIP telephony) using non-persistent communications. Clients issue periodic heartbeat messages to a connectionless server using a non-persistent (e.g., UDP) communication protocol. The heartbeat messages identify the clients by unique client identifiers. When an application server has an unsolicited communication for a client (e.g., a new instant message, a new VOIP call), the application server retrieves the client's address from the connectionless server if it does not already have the address. The application server forwards the unsolicited communication to the client via a non-persistent communication, and may spoof the connectionless server if necessary to reach the client through a firewall, proxy, NAT or similar entity. Clients that cannot use non-persistent protocols establish persistent connections with a client manager. The client manager issues heartbeat messages to the connectionless server and receives and forwards unsolicited communications for these clients.