International caller IDs can be verified using Twilio, but your project must have permission to dial the country in which your phone number is located. To do this you will need to visit the Global Permissions page for Voice and enable the country where your phone is based.
If you've visited the Global Permissions page for Voice and the country you are looking for is not listed, you might need to upgrade your project and request international dialing permissions.
After your project has been enabled for international dialing, please visit the Global Permissions page for Voice again. You should now have the ability to enable outbound calls to any country in the world, which means that the verification call can go through. Please enable the country you wish to call.
Once your project has permissions to dial the phone number you want to verify, you can either click the "Verify a number" button on the Verified Numbers page, or use our REST API to verify individual caller IDs programmatically.
If you have difficulty durring the verification process, you may want to read this article on E.164 phone number format to be sure you are inputting the phone number in a format which Twilio understands.