I can’t make phone calls on my iPhone

To be able to make cellular calls while on Wi-Fi, your service provider must support Wi-Fi calling, and your service plan must have the feature enabled on it.

See here: link-> Make a call with Wi-Fi Calling - Apple Support

Please check this link on how to enable Wi-Fi calling. It helps when you have a week cellular connection. If this does not help, please be give us more information about your phone.

https://support.apple.com/en-us/HT203032