How to build an XML file fior Twilio through a URL in a formula field:

Use Case: 
You need to build a dynamic text-to-voice concatinated with different audio urls (audion hosted on a website). To accomplish this you need to dynamically build a XML url in a formlua field with audio or voice to text.

•  What I did was anaylize how Twilio was doing with thier Twimlets https://www.twilio.com/labs/twimlets
• Create field for voice-to-text message (will represent Say)
• Create field for audio url (will represent Play)

Key factors to consider in your formula

• No blank spaces alowed in the url 
  • Use %20 to represent a blank space
• audio urls always are treated as "Play"
• Message%5B0%5D=[text or url here] is template for seperating the text and audio urls (more on this later)
• text is always treated as "Say"
• %3A = :
• %2F = /

Example Working Formula:

'http://twimlets.com/message?Message%5B0%5D='&SUBSTITUTE(Subject, " ", ",%20")&'.%20'&Contact.LastName&'.&Message%5B1%5D='&SUBSTITUTE(SUBSTITUTE(Url__c,':', '%3A'),'/', '%2F')&'&Message%5B2%5D=%20'&Id&'&'

Formula result:

http://twimlets.com/message?Message%5B0%5D=Thanks,%20for,%20checking,%20this,%20out.%20Houser.&Message%5B1%5D=https%3A%2F%2Fpink-dog-6501.twil.io%2Fassets%2FSampleAudio_0.4mb.mp3&Message%5B2%5D=%205001I000004ffiT&

XML created:

<Response>
<Say>Thanks, for, checking, this, out. Houser.</Say>
<Play>
https://pink-dog-6501.twil.io/assets/SampleAudio_0.4mb.mp3
</Play>
<Say>5001I000004ffiT</Say>
</Response>

What this formula does:

• 'http://twimlets.com/message?Message%5B0%5D='&SUBSTITUTE(Subject, " ", ",%20")&
  • twimlets.com check it out
  • Message%5B0%5D=
    • ​Notice the '0', this indicates first position
  • '&SUBSTITUTE(Subject, " ", ",%20")&
    • Replaces blank spaces with an URL acceptable term = %20
  • '.&Message%5B1%5D='&SUBSTITUTE(SUBSTITUTE(Audio_URL__c,':', '%3A'),'/', '%2F')&
    • (.) pauses 
    • SUBSTITUTE(SUBSTITUTE(Audio_URL__c,':', '%3A'),'/', '%2F')
      • the audio url 
      • This replaces ";" and "/"  with the acceptable url terms
  • '&Message%5B2%5D=%20'&Id&'&'
    • I ended the call with another text field called Id

Other notes: 
Message%5B1%5D = Message[1]
Message[0] The URL of a media file to <Play>, or a string to <Say>

Check out the Twimlets from Twilio here --> https://www.twilio.com/labs/twimlets