Enable Wi-fi Calling
Sometimes LTE voice data is exactly what you need to improve your connection. Other times, the data connection may be poor no matter how fast it’s supposed to be. In situations like these, try switching to a Wi-Fi connection instead.
You can find Wi-Fi calling as an option in the Cellular section of your iPhone’s Settings.
If you have a strong, nearby Wi-Fi signal, this may help improve your calls dramatically (and you won’t use up as much of your data allotment). Although, not all carriers include free Wi-Fi calling as part of their wireless plans.