Good Morning MM-0329. It seems very strange that the phone just flips to roaming, unless you're in an area that the coverage is weak. Cell towers are a lot like elevators, meaning they have load limits as well. If a tower is overloaded, calls could be dropped, or phones may slip into roaming if there are no other towers in the area to help carry the load.
May I have a couple cross streets and the zip code so I may look into the towers in this area?
Cat Small Scale Farmer Pulling issue and planting solutions. If I was able to plant your solutions, please accept and/or kudos the resolution.