There is NOT a cap that twilio has instituted for outbound messages. There has been for quite some time, bans on too many SMS across the individuals carrier lines. You would have to send quite a few messages all at once on a single carrier to have this happen and more than likely will see the errors come up in twilio about this. But twilio is not going to cap anyone or has no plans to, They suggest keeping it down to 250 messages per carrier but that is just a suggestion. You can verify this yourself by asking twilio support if they have instituted a cap and they will tell you similarly.