How can I make a numeric keyboard appear with a regular field when form is filled on a mobile device?

When I create a form and  I use the phone field under basic form elements, it automatically shows a numeric keyboard if you are filling the form on a mobile device.

How can I make this numeric keyboard appear with a regular field?

When you say a 'regular' field, are you referring to a Short Text Entry (Text Box) field?

It is not possible to show only a Numeric keyboard with a Short Text Entry field. The Alphanumeric keyboard will always be shown, even if the validation of the field is set to Numeric.

If you want that a Numeric keyboard is shown, you should use a Number field.