Well I think Whisper is more of a social sharing site while this tool is about sending anonymous messages to a specific target number. As for the stack it is Rails(which was probably overkill but I have yet to do much with Sinatra), Twilio, with Stripe integration. I send the messages via Twilio and then use their API to redact the logs so I can't see what people are messaging using the service.
Based on the anonymity aspect, I was thinking of Whisper Systems: https://whispersystems.org/ Hence my curiosity about the stack.