Twilio Messaging Services use Scaler to spread your outbound messaging traffic evenly across all phone numbers in your Messaging Service. When a message is sent using a Messaging Service SID, the
From number is automatically selected from among the numbers not currently in use in your number pool.
When a short code is included in your Messaging Service, Twilio will prioritize the short code for its geographic region above any other long codes. For example, if a US short code and US long code is available in the same Messaging Service, the US short code would always be prioritized first.
For full details, see Messaging Services - Scaler.