What types of phone numbers can I not dial?

Twilio does not support outbound calls that are made to shared cost routes as well as premium rate telephone numbers.

Shared cost routes are phone numbers where the charge for the call is partially paid by the recipient of the call. Premium rate telephone numbers are numbers which are priced higher than normal. When premium numbers are called they enable funds to be transferred to the caller and allow the caller or recipient to offset the cost of their call.

When calls are made to these types of numbers, the calls will fail and an error will be shown in your debugger logs.