Why doesn't the mobile phone support both the primary and secondary cards to use the telecom card?

Patent of telecommunication CDMA technology is in Qualcomm's hand, including 2G CDMA 1x and 3G CDMA EVDO. Although telecom 4G is not CDMA, if you want to make a phone call or send a text message, you still need to use CDMA network. Besides producing SoC chips, another main source of profit for Qualcomm is baseband expenses. Every time a mobile phone manufacturer sells a telecom mobile phone, it has to pay a patent fee to Qualcomm. Because this fee is calculated according to the baseband, if it is a dual telecom baseband, it will give Qualcomm twice the patent fee, which greatly increases the production cost of mobile phones.

The 2G GSM patents used by China Mobile and China Unicom are not from Qualcomm, nor are the 3G TD-SCDMA and 4TD-LTE developed by Qualcomm. Although TD-LTE technology also involves some Qualcomm patents, the cost after cross-licensing is much lower than that of CDMA patents. Finally, although Qualcomm holds the patent of FDD-LTE, Qualcomm actively reduced the patent fee of FDD-LTE in order to compete with TD-LTE technology. Therefore, the current mobile phones of Netcom can be mobile or Unicom with dual cards at the same time. Only telecommunications can't have dual cards at the same time, because CDMA patents are too expensive.