It depends on what is the type of your Phone field in Marketo. If you are using the built in Phone field, it is of type 'Phone'. We use the same. And that lets our prospects to submit data as 18999434343x4343 etc.
You might be using a custom field of some different type. You can check that in Admin section in Marketo.
Or
you might be having custom javascript validation code on your forms only allowing digits etc.
Depending on your case, you can just work around the field type or adjusting validation code rather than having to add a separate field.
Hope this helps
Rajesh