IMHO you need to use a rule for that like:
rule "MyMQTTRule
when
Item Smartphone changed
then
val mqttActions = getActions("mqtt", "mqtt:broker:MQTTBrokerTest")
mqttActions.publishMQTT("MyTopic", Smartphone.state)
end
Please insert your desired Topic instead of “MyTopic”. Could not test ATM, so I’m not sure if the part “Smartphone.state” is correct.