Verizon World Plan - Calls to Philippines blocked, but can call using other providers

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We recently added Verizon World Plan to a regular (copper/non-FIOS) residential line, but calls to Philippines are being blocked with the message "We're sorry, calls to this area are blocked  VH005."  We can successfully dial the affected numbers directly with a T-Mobile cell phone and also using third party services like Startec, and have verified with Verizon customer servcie that we do not have an account block. Very frustrating, since we bought this plan specifically to call these numbers.