ResponsiveVoice will try to speak a very big number in its entirety, which in words surpasses the limits of the TTS engine.

As an example, the number

  

777,777,777,777,777,777,777,777,777,777

 

would be read as 

  

seven hundred and seventy seven octillion 
seven hundred and seventy seven septillion 
seven hundred and seventy seven sextillion 
seven hundred and seventy seven quintillion 
seven hundred and seventy seven quadrillion 
seven hundred and seventy seven trillion 
seven hundred and seventy seven billion
seven hundred ans seventy seven million 
seven hundred and seventy seven thousand 
seven hundred and seventy seven 

 

A common workaround is to simply write the number out, instead of using its numerical form, as the TTS engine will interpret it correctly.