Re: why is my I-phone 6 dropping coverage constantly?
Another thing to check is go to Settings -> About and your phone will check for the latest carrier settings. If there is a new one it will ask you to update.
Otherwise you might try rebooting your phone by holding the power button and home button down simultaneously until you see the Apple boot logo.
If neither of those help, then I'd say you're in a fringe coverage area that will be spotty unfortunately.