I never play with twilio but i know sapi_SPVoice (text to speech).
for fun try this code

* load SAPI Engine
loVoiceEngine = CREATEOBJECT("SAPI.SpVoice")
if !vartype(loVoiceEngine)="O"
messagebox("Sapi.SpVoice not installed...cancelling",16+4096)
return .f.
endi

* speak a text
text to m.myvar noshow
this is a testof text to speech with sapi.SPVoice activex control 
under microsoft visual foxpro.
can speech any text or play any wav music file.
you can choose another speaker voice and set a language as english,french...
endtext
loVoiceEngine.speak(m.myvar)

* list installed voices
local m.lnindex,m.xx
m.xx=""
m.lnIndex = 0
FOR EACH loVoice IN loVoiceEngine.GetVoices
lcDescription = loVoice.GetDescription()
m.xx=m.xx+ lcDescription +"  "+trans(m.lnIndex)+chr(13)
lnIndex = lnIndex + 1
ENDFOR
messagebox(m.xx,0+32+4096,'',2000)

* change voice voice 
loVoiceEngine.voice = loVoiceEngine.getvoices().item(1)
loVoiceEngine.speak(m.myvar)