TTS does not allow you to specify how the text should be read so your code has to modify the text input so that it reads the individual numbers.
I suggest @opalenzuela's mention that a space in between each number. That should cause the TextToSpeech to read the individual numbers.
If you need some code to help you detect numbers use this:
private boolean isNumber(String word)
{
boolean isNumber = false;
try
{
Integer.parseInt(word);
isNumber = true;
} catch (NumberFormatException e)
{
isNumber = false;
}
return isNumber;
}