This could be a glitch in the firmware and the iPhone network settings needs to reset.
Please follow these steps>
Step 1:Tap Settings.
Step 2:Tap General.
Step 3:Scroll down to the bottom of the page and tap Reset.
Step 4:Tap Reset Network Settings.
Step 5:You will get a prompt warning you that this option will delete all network settings and return them to factory default settings. Tap Reset to continue.
Step 6:Wait while your iPhone resets its network settings.
Once the reset process is done, your network settings will return to its factory default settings. Your iPhone will also automatically restart. Then check.
did that work for you?