In Massachusetts, the sales tax on a cellphone that comes at a deep discount when someone signs a contract isn’t based on what a customer actually pays for the phone, but on its higher wholesale price. Now, as millions of people buy pricey smartphones, an obscure tax directive created at the beginning of the wireless market is suddenly getting a lot more attention. Not only are consumers voicing their surprise - and often outrage - at the register, state lawmakers have introduced bills to change the policy.