T-Mobile announced expanded coverage on Thursday of its iPhone-compatible HSPA+ network in five metro areas, including Chicago, Reno, Fresno, Sacramento and Southern California. The company’s improved network is now available in 23 markets across the country and covers 100 million people. T-Mobile notes that because the network operates on the 1900MHz frequency, it encourages AT&T (T) customers to switch over and use their unlocked smartphones, including the iPhone, with the carrier. The company also said that due to overall “network advancements” in areas such as Boston, Los Angeles, New York, Philadelphia, and San Diego, customers in these areas should already be experiencing improved coverage when using unlocked devices.