The Phone Number checkout field allows using text characters. How can I change it?

The Phone Number checkout field allows using text characters. How can I change it?

One Step Checkout Pro allows you to change the validation type of the Phone Number field directly in the configurations. For that, proceed to Stores → Settings → Configuration → Amasty Extensions → One Step Checkout → Additional Fields and Other Extras and switch the Phone Number Field Validation field to Numeric Only: