ResponsiveVoice not working when page load

Question

Why this code works well on my Android Tablet (Samsung) + Desktop Windows 10 , but not on my 2 Android Phone (LG4) and Samsung 3

<script>
   setTimeout(responsiveVoice.speak("Welcome to the Responsive Voice  website"),15000);
</script>

Calling this function has exactly the same problem (works on tablet but not on phones):

function x() {
                responsiveVoice.speak("Hello");
             }
function y() { 
             x();
           }
y();