Sorry for the delay on getting to you about this. Just so I can get an idea of how to help, are you driving through this area or at a home or work location? The coverage does taper off a bit as you go south on US-21 but along I-26 the coverage appears to be more solid. Of course this is just me looking at the general view of the area. In order to get a something a bit more accurate, can you look up the address in question using our coverage map and tell us what kind of coverage it's showing?
You could try switching to 2G on your phone, giving it a restart and see if that connection helps with the calls. Depending on the signal you're getting in that area, you may not need to keep it on 2G but it's worth a shot testing this setting out. Also, you're phone has Wi-Fi calling so if you have an internet connection and Wi-Fi router to connect to, you can make calls over that to see if it helps.
We'll do everything we can to help so please keep us posted on this and let us know if you need more assistance.