Apple's dual-network telecom card can't send text messages

Because your mobile phone lacks CDMA network support. The FDD of telecom can bring data to the Internet, but it does not support SMS and voice. In order to realize voice and short message functions, telecom must have CDMA network. According to your description, your American version of iPhone7P can surf the Internet but can't make phone calls. It should be because your mobile phone lacks CDMA network support. If you want to realize the telecom voice function, you can consider opening the telecom VOLTE, so that you can make calls under the 4G network.

Code Division Multiple Access (CDMA) is a new and mature wireless communication technology developed on the basis of spread spectrum communication technology, a branch of digital technology. The principle of CDMA technology is based on spread spectrum technology, that is, the information data to be transmitted with a certain signal bandwidth is modulated by a high-speed pseudo-random code with a bandwidth much larger than the signal bandwidth, so that the bandwidth of the original data signal is expanded and then transmitted by carrier modulation. The receiver uses the same pseudo-random code to correlate with the received bandwidth signal, and changes the broadband signal into the narrowband signal of the original information data, that is, despreading, thus realizing information communication.

CDMA refers to spread spectrum multiple access digital communication technology, which establishes a channel through a unique code sequence. This technology can be used in any protocol of the second and third generation wireless communication. CDMA is a multi-channel mode, and multi-channel signals only occupy one channel, which greatly improves the bandwidth utilization. It is suitable for 800MHz and 1.9GHz UHF mobile phone systems.

CDMA uses analog-to-digital conversion (ADC) with spread spectrum technology, and the input audio is first digitized into binary elements. The frequency of the transmitted signal is coded according to the specified type, so only the receiver with consistent frequency response coding can intercept the signal. Because there are countless kinds of frequency sequence codes, it is difficult to repeat them, which enhances the confidentiality.

The CDMA channel width is labeled as 1.23MHz, and the soft handover scheme is adopted in the network to minimize the signal interruption during the mobile phone call. The combination of digital and spread spectrum technology makes the number of signals per unit bandwidth double that of analog mode, and CDMA is compatible with other cellular technology, thus realizing national roaming.