The problem is Twilio provides no way of billing end users. In this case it comes down to a micropayments problem (which still haven't been solved sufficiently)

The problem is Twilio provides no way of billing end users. In this case it comes down to a micropayments problem (which still haven't been solved sufficiently). So you either have to charge up-front for a monthly service fee of a reasonable sum, or stick adverts everywhere which will hopefully cover your costs. It's a major flaw for the platform if you want to build a business off of it (I've been playing around with a per-monthly-fee service concept recently).