Some of my friends calls have been routed to voicemail, even starred and frequently contacted numbers. It seems very random in that sometimes they can reach me and other times it gets routed automatically. I checked my contacts list and (Google contacts list) and there is a setting route to voicemail/unroute to voicemail. Starred contacts display unroute to voicemail but this is still happening
If they happen to call when your phone signal either has interference or lowers, calls will go to voicemail. Unless you always have eyes on your phone signal, you won't know when this happens. You can even have 5 bars, but there could be network interference on the carrier end that prevents the call from getting to your phone. You should contact your carrier. As for the route/unroute to voicemail - that's a phone setting and if the calls are not even getting to your phone due to carrier issues, it's not likely to have any effect.
