This can be done with an input mask in a single line text field.
If you configure your fields “Restriction” settings like this: https://goo.gl/EYiC27
Then the front end of your form will accept ONLY numbers and look like this:
https://goo.gl/9QjDXM
I hope this helps!
Because we only check these forums once per week, we strongly encourage you to reach out to us here with any followup questions in order to get a timely response: https://ninjaforms.com/contact/ There, we offer support free support directly to our Ninja Forms users with significantly more privacy and a faster response time.