To verify new outgoing caller ID numbers, you can either use the Console or the REST API.
To use your Console, log into your Twilio project and visit the Verified Caller IDs page. Click the + button at the top of the page.
To use the REST API, POST your phone number to the Outgoing Caller IDs list resource.
All numbers purchased from Twilio or ported over to us are already verified as caller IDs.
Verifying Caller IDs Behind an IVR or Extension
You can verify numbers behind IVRs using both the above methods by including ‘w’ characters in the Extension parameter. Each ‘w’ character tells Twilio to wait 0.5 seconds instead of playing a digit. This lets you adjust the timing of when the digits begin playing to suit the phone system you are dialing.
For example, the extension “wwww2wwwwww5wwwwww9″ waits two seconds before sending the digit 2, followed by a three second wait before sending the 5, and finally another three second wait before sending a 9. You will need to tune the extension to the IVR system to be able to programmatically verify numbers behind IVR menus.