Calling from an anonymous number (Where the receiver see "No Caller ID" or "Anonymous" in the display instead of a origin phone number) has long been a bad practice. People do not pick up such calls and it's increasingly being used to flag calls as spam in the telephony networks. Furthermore prices recently started differing, meaning it's more expensive to call e.g. EU countries if you call without a Caller ID.
We found that in most cases, when customers are calling without a Caller ID, it's a mistake. So by blocking such calls, we ensure that users follow best practices to optimize their connection rate AND get the best rates possible.
To assign a Caller ID, first make sure you have one available under Manage -> Voice, the assign it in the user preferences under Dialling. For higher-tier plans, Caller IDs can also be assigned to lists in the list configuration page under Settings.
Read the full tutorial on adding Caller IDs.