Twilio phone service retry outbound calling just once
This problem happens when using Dial verb as well and it is very annoying that Twilio documentation doesn't mention this continuous retrying. There has to be a way to prevent retries as like you say it is a show stopper for a lot of situations.
Here is a link to pretty much same issue on Twilio forums: http://forum.twilio.com/twilio/topics/outbound_calls_keep_incessantly_retrying