On your ingress of receiving messages, you'd need to have something that parses their text and call on a different function. Having some sort of AI here could help improve the results, but that's definitely a beyond the scope of this content. 

In this particular case, you could change

case body.upcase.strip

to something like

case
  when body.upcase.strip.include?('MENU')

so, each switch in the case statement would need to have the conditional check.