Hi Cindy. Sounds like Sprint might have a bit of a hollow coverage spot to that area you travel often? Coverage can vary depending on the location of the towers. Here is a link for Sprint's coverage maps. Have a zoom in and see if they claim coverage for that area. This tool from OpenSignal is also very helpful to determine what coverage is available in your area and the areas you travel to often.

Your next step would be to contact Sprint (customer service number for Sprint here) and ask them about the problem. They can advise you on your options and whether or not you should have coverage at the locations you experiencing dropped calls.

One other thing to note is that if you've signed a new contract with Sprint and the coverage in your area is not suitable, you may be able to get out of your contract because of it.