That’s exactly what would happen to me when I had little to no cell signal & the signal was very unstable & changed constantly. Sounds like this is def a carrier issue. Call your provider when you’re wifi connected, wifi calling set to on & ask for level 2 Tech support. Give them your # & ask them to call you right back if call’s dropped (they should be able to reach you if you’re wifi connected & wifi calling is on). Eventually, AT&T told me the problem was the topography between my home & their towers. They said, ‘We don’t guarantee service everywhere’ which is a hard pill to swallow when you’re paying $160/mo. I ended up switching carriers & that solved my problems.
You’ve prob already done these but also try resetting your network settings, turn airplane mode on then off & reboot your phone.