521: You can only use your RM-917 device on the WCDMA 850, 1700 (AWS), 1900 and GSM/EDGE 850, 900, 1800, 1900 MHz networks. You need a subscription with a service provider 520: You can only use your device on the WCDMA 850, 1900, 2100 and GSM/EDGE 850, 900, 1800, 1900 MHz networks. You need a subscription with a service provider 我也下单521了,以上信息出自官网的manual, 联通的wcdma是2100,所以521估计只能用中国移动或联通的2.5g了