callbacks.qml

DialogPage.qml

Page {
    id: page
...
property var callback
...
// some place where you need callback:
page.callback("some", "very", "important", "params")
}

OtherPage.qml

Page {
...
function callbackFromDialog(param1, param2, param3 ...) {
// do something with params
}

// some place where you pushing page:
pageStack.push(Qt.resolvedUrl("DialogPage.qml"), {callback: callbackFromDialog })

...
}