byte[] sendData(byte[] terminalData) is the API to send XML message to the phone. This API is available on CiscoTerminal.
You can put the following as a byte[] and use the above API.
<CiscoIPPhoneText><Title></Title><Prompt></Prompt><Text>This is a test message through sendData</Text></CiscoIPPhoneText>
JTAPI test tool available at Cisco JTAPI show cases this functionality as well. You can use this test app to get familiar with Cisco JTAPI API.