How does the flow card block incoming calls and messages?

Cell phone signal screener can screen all 2G, 3G, 4G and 4G signals of China Mobile, China Unicom and China Telecom. Calling and texting mainly take 2G frequency, and surfing the Internet mainly takes 4G frequency. Although the mobile card you bought doesn't have a phone number, you still use the mobile phone frequency at work, which is also within the shielding range and will be blocked. Your mobile phone needs a firewall. Incoming call firewall and SMS firewall. Set the number to be blocked to the blacklist of the firewall. When the other party dials, it will prompt that it cannot be connected, send you a message, and you will also receive a report of sending failure.