This is a great question! Generally speaking we recommend for roaming to be turned on. There are some areas that are covered by our Extended Network. This means we have agreements to share towers with other providers. With Roaming turned off your phone won't connect to shared towers - only ones exclusive to Verizon. There are instances where we do not recommend turning roaming on, though. Usually if you are close to a boardering country we recommend having it off to avoid Roaming charges if your phone accidentally connects to a cell tower outside of the USA.
Also, what services aren't working in these areas? What zip codes are you in when this happens? We'd love to check what the expected coverage is like for you.
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All I know is I had no phone service or data service in the zip code 46947. When I turned on voice roaming I then had what said Verizon extended. I didn’t use my phone at that time so presume everything was ok.