cucumber_example.rb

class SMSTester
  def initialize
    @number_of_sms = 0
    @number_expected = 0
  end

  def should_have_received_message_containing
    @number_expected += 1
    # do something that might send an SMS
    @number_of_sms_sent += # introspect this somehow?
  end
end

Given /I do something that sends an SMS/ do
  @sms_tester = SMSTester.new
  @sms_tester.do_something
end

When /I do something again/ do
  @sms_tester.do_something
end

Then /I should receive an SMS containing blah blah/ do |msg|
  @sms_tester.should_have_received_message_containing(msg)
end

Then /I should not have received another SMS/ do
  assert @sms_tester.number_of_sms_sent < @sms_tester.number_expected
end