gistfile1.rb

## Copyright 2009 Rex Luther Corporation.
## Licensed under the MIT License
## http://www.opensource.org/licenses/mit-license.php

def valid_phone
  return true if phone.blank?

  phone_formats = [
    /^\(\d\d\d\) \d\d\d-\d\d\d\d/,
    /^\d\d\d\.\d\d\d\.\d\d\d\d/,
    /^\d\d\d\-\d\d\d\-\d\d\d\d/
  ]
  
  valid = false
  phone_formats.each do |format|
    if phone.match( format )
      valid = true
    end
  end

  unless valid
    errors.add("Phone format isn't recognized")
  end
end

No need to loop and have separate regexes. You can use the | operator to permit multiple formats

/^\(\d{3}\)\s?\d{3}-\d{4}|((\d{3}\.){2}\d{4})|((\d{3}\-){2}\d{4})$/g

http://refiddle.com/gv3