Field Formats: How to accept numbers only?

How do I get a numeric format only for phone numbers and other fields? So alphabetic use is not allowed. 

You may use the fixed format widget: https://widgets.jotform.com/widget/fixed_format

This guide will help you adding it to your form: https://www.jotform.com/help/252-How-to-Add-a-Widget-to-Your-Form 

You may use the input mask feature, more details on this guide: https://www.jotform.com/help/381-Getting-Started-with-the-Basics-of-Input-Masking 

You may also use a short text entry element and set a numeric validation, the option can be found under the field option settings: 

I hope this helps.