and anticipation. Upon its release, it was marketed heavily in television, web and print ads created in partnership with TBWA\Chiat\Day. Apple's premium market positioning has led the iPhone to be seen as a status symbol. The Apple ecosystem has been described as a key moat that increases iPhone brand loyalty. iMessage has especially been singled out, with its "green bubbles" phenomena. In iMessage, SMS messages from Android users appear as green bubble, rather than the blue bubbles used for texts from other iPhone users. Group chats between iOS and Android are poorly supported; reactions display as text, rather than bubbles, and images are sent through MMS, which degrades image quality. Some teens have described being "ostracized" after switching to Android, which Google has labelled "bullying." This has been described by critics as a key factor leading 87% of U.S. teenagers to use iPhones. Retail SIM unlocking Many iPhones bought through a monthly carrier contract are SIM locked, restricting their use to one particular carrier. While the iPhone was initially sold in the U.S. only on the AT&T network with a SIM lock in place, various hackers found methods to bypass that SIM lock. More than a quarter of first-generation iPhones sold in the U.S. were not registered with AT&T. Apple speculated that they were likely shipped overseas and unlocked, a lucrative market before the iPhone 3G's worldwide release. Today, many carriers either remove the SIM lock automatically after a certain period, or do it upon request, either for free or for a small fee. iPhones bought from Apple are not SIM locked. Many carriers also sell the iPhone unlocked when purchased outright rather than on a long-term contract. Retail strategy Since 2013, iPhone buyers can obtain a trade-in discount when buying a new iPhone directly from Apple. The program aims to increase the number of customers who purchase iPhones at Apple Stores rather than carrier stores. In 2015, Apple unveiled the iPhone Upgrade Program, a 24-month leasing agreement, which Fortune described as a "change [in] iPhone owners' relationships with mobile carriers". Repairability Only Apple Stores and Apple Authorized Service Providers are allowed by Apple to perform genuine replacements. Apple has taken steps to make third-party repairs more difficult. iPhone components are soldered, and many are glued together. iPhones receive low repairability scores, in part due to the difficulty of obtaining genuine parts, and the difficulty undertaking each repair. This has