This is a customer-to-customer forum; have you tried using another phone and contacting Customer Service?
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Then my guess would be a tower issue. If you want to know for sure, again, you'd need to contact Customer Service. Verizon reps pop into the forum and answer questions, but this thread isn't marked as a question (and even if it were, it's sometimes days, if at all, before a Verizon rep responds).