[Plugin: Fast Secure Contact Form] Input Mask Possible?

yes you can use the regex feature for this
as described on the extra field help page
https://www.fastsecurecontactform.com/how-to-add-extra-fields

Use to validate if form input is in a specific format. Example: If you want numbers in a text field type but do not allow text, use this regex: /^\d+$/ Can be used for text, textarea, date and password field types.

here is a pattern you can use for telephone numbers in the US
/^(\(\d{3}\)[- ]?|\d{3}[- ])?\d{3}[- ]\d{4}$/

Description
US telephone pattern. Optional area code with or without ‘()’. If area code has parentheses then it is optionally followed by – or space. If no parentheses then it must be followed by – or space. Number must be separated by either a space or -.
Matches
(555)555-5555 | (555) 555-5555 | 555-5555
Non-Matches
5555555555 | 555.555.5555 | (555 555-5555

Thanks, That helps a lot!