initializer_arguments.rb

class InitializerArgumentsPaymentProcessor
  def initialize(number, expiration_date, amount)
    @number, @expiration_date, @amount = number, expiration_date, amount
  end
  
  def call
    if SomeGateway.(number, expiration_date, amount)
      true
    else
      false
    end
  end
  
  class Substitute
    def call
      @approved
    end
    
    def approve
      @approved = true
    end
    
    def decline
      @approved = false
    end
  end
end

instance_method_arguments.rb

class InstanceMethodPaymentProcessor
  def call(number, expiration_date, amount)
    if SomeGateway.(number, expiration_date, amount)
      true
    else
      false
    end
  end
  
  class Substitute
    def initialize
      @approved = []
    end
    
    def call(number, expiration_date, amount)
      @approved.any? do |n, e, a|
        n == number && e == expiration_date && a == amount
      end
    end
    
    def approve(number, expiration_date, amount)
      @approved << [number, expiration_date, amount)
    end
    
    def decline(number, expiration_date, amount)
      @approved.delete_if { |n, e, a|
        n == number && e == expiration_date && a == amount
      end
    end
  end
end

Note that for the initializer arguments example, a class level call method can be added to give the class the same interface as instances of the instance methods arguments example. So you get both approaches:

class InitializerArgumentsPaymentProcessor
  def self.call(number, expiration_date, amount)
    instance = new(number, expiration_date, amount)
    instance.()
  end
end