I appreciate you taking the time to troubleshoot with us, BRICAR5. I have sent you a Private Message and I will look forward to investigating the network records further to determine what is happening with these calls.
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Having FiOS service doesn't necessarily indicate Internet-based calling. I have Verizon FiOS here at home. My telephone calls are processed by Verizon (Ma Bell)'s Lucent 5ESS switch in my town.
How is the signal strength at the time this happens? If the system thinks that the subscriber offline, calls will get dumped into voicemail.