golf.rb

[ :a, :b, :c ].inject( [ [] ] ) { |arr, n| arr.push( arr.last.dup << n ) }.reverse
# => [[:a, :b, :c], [:a, :b], [:a], []]

class Array
  def countdown
    inject( [ [] ] ) { |arr, n| arr.push( arr.last.dup << n ) }.reverse
  end
end

query = {
  :country_code => user.country_code,
  :region_code  => user.region_code,
  :zip_code     => user.zip_code
}

[ :country_code, :region_code, :zip_code ].countdown.find do |constraints|
  number = TwilioNumber.shared.available.where( query.slice( constraints ) ).first_or_create
  number.valid?
end
# >> [:country_code, :region_code, :zip_code]
# >> [:country_code, :region_code]
# >> [:country_code]
# >> []