My carrier’s been great, but I regularly visit a location where it’s Verizon, AT&T, or no service. To maintain texting continuity, I plan to get a 2nd phone, plug in an MVNO SIM, & use as hot spot only. I’ll need almost no internet access with this setup, but don’t know how to estimate how much data my texting & 10 hrs/mo of talk time will translate to.
Phone should have a program to track data usage. Use the phone for an hour, multiply by whatever other time you want to check it for. The phone does not really use "data" for texting or calls unless you are attaching images to texts or are using WiFi calling. The data plan is not used for calls.