Using Twilio test credentials allows simulation of sending SMS messages from/to valid numbers, non-SMS-capable numbers, invalid numbers, etc., by using specific numbers for the value of the 'From'/'To' parameter of the HTTP POST to Twilio.
Is there any way to use Twilio test credentials to simulate sending (successfully or unsuccessfully) using a value for the 'MessagingServiceSid' parameter instead of a value for the 'From' parameter?