QQ.com reports (rough translation):

Beijing Chaoyang District police have warned people to be very careful about returning missed calls from numbers starting with 0941 or 0951. According to reports from citizens, returning such calls incurs a mobile phone fee of 500 yuan. 0941 is the area code for a place called Hezuo Shi (Cooperation City!) in Gansu.

0951 is the number residents of Yinchuan, Gansu dial to hear weather reports. According to the police, 0941 and 0951 are the first digits of fee-based phone services, so be careful about phoning any such number.

Other Danwei sources say that phone numbers starting with 0271 are also being used regularly for mobile call-back scams this summer.