Hi @Velez, Andres , we don't have an exact list of suported phone numbers, just that they'll need to provide phone numbers in E.164 international standard format. For example, +14255550123. The Toll Free Verification page also has some more details. This document has troubleshooting steps for these issues. Please walk through them and let me know if it helps.
If these steps don't work please let me know the following information: What are the errors your students are running into? Also, could you provide an example of a numbers and the countries they're from?
If this answer helped you please mark it as "Verified" so other users can reference it.
Thank you, James