Verizon is aware of an international third-party communications provider having issues with voice and data connections with U.S. based customers traveling overseas. They are actively working on fixing these issues.
We recommend using Wi-Fi when available. If you had previously activated Wi-Fi Calling back in the United States, that feature can be used as well in the meantime: https://www.verizon.com/support/wifi-calling-faqs/
We appreciate your patience and understanding, and thank you for using Community Forums to lift your feedback and experience.