@bone_naga: You do NOT have to contact your U.S. cellular-service provider to “ask why it works for other providers in other countries and not yours.”
All U.S. cellular carriers have followed voicemail-notification and -access standards in the USA for more than two decades.
Here is more about how the notification standard works and what Mudita had to implement in MuditaOS K 1.6.0 to meet this standard: