Modern smartphones are almost always equipped with slots for two SIM cards, which allows users to flexibly manage calls and mobile data. However, owners of devices based Android often encounter a situation where one of the slots simply stops seeing the card, or the network disappears for no apparent reason. This causes natural confusion and is often attributed to hardware failures, although in most cases the problem lies in software settings or hardware compatibility.
The concept that the second SIM card is not working can range from complete loss of signal to the inability to make a call when the Internet is active on another card. Before taking the gadget to a service center, it is necessary to conduct a thorough diagnosis. In this article we will analyze in detail all possible scenarios, from a banal rearrangement of cards to complex conflicts frequency ranges and problems with IMEI.
The situation when the phone sees only one card often occurs after updating the operating system or resetting the settings. Less commonly, the culprit is physical damage to the contacts or the use of adapters for nano-SIM, which displace the card in the slot. Understanding the operating principle of radio module your smartphone will be the first step to successfully restoring communication.
Hardware reasons and checking the physical connection
The first thing that needs to be ruled out is a physical malfunction of the SIM card or slot itself. Often users blame the phone, while the problem lies in the plastic of the card, which wears out or oxidizes over time. Try moving the problematic SIM card to the first slot. If it works, then everything is fine with the card itself, and you need to look for the reason in the settings of the second slot.
The use of adapters from micro-SIM to nano-SIM is a common cause of unstable contact. The thickness of such a “sandwich” may be slightly larger than normal, which is why the card does not fully fit into the tray or puts pressure on the contacts of the adjacent slot. In some smartphone models, especially budget ones, the second slot is hybrid and supports either a second SIM or a memory card MicroSD. Make sure that you have not inserted a USB flash drive instead of a SIM card.
⚠️ Attention: If you are using an adapter, make sure that it is no thicker than a standard nano-SIM. Excessive thickness can lead to jamming of the tray or damage to the contact group inside the phone.
Inspect the contacts on the SIM card itself. They must be clean, without scratches or signs of corrosion. Wipe them with a soft cloth soaked in alcohol. Also check the smartphone tray for dust or debris that could prevent the card from sitting snugly. Sometimes careful cleaning of the slot with compressed air helps.
☑️ Diagnostics of the physical connection
Software settings and SIM card management
In the operating system Android SIM card management is placed in a separate menu, where the user can manually disable slots. This often happens accidentally when changing the theme or after a firmware update, when the settings are reset to factory settings. Go to the section Settings → Connections → SIM card manager (the path may differ depending on the shell, for example, MIUI or OneUI).
Make sure the switch opposite the second SIM card is active. If it is turned off, the phone will not even try to register with the operator's network. Also check your roaming settings. If you are in the coverage area of your operator, but the phone defines the area as roaming, and the “Data Roaming” option is turned off, the Internet will not work, although voice communication may remain.
In some Android shells (for example, older versions of MIUI) there is a “Fast SIM switching” function, which can automatically turn off the second card when the signal is weak to save energy. Try disabling this option.
Another important parameter is the choice of network. In the settings of the second SIM card, find the item “Network type” or “Preferred network type”. If “4G/LTE only” is selected there, and there is no or unstable 4G coverage in your area, the SIM card will show the “No network” status. Switch the mode to 2G/3G/4G (auto)so that the device can cling to available towers.
Conflicts of operating modes and frequency ranges
Modern smartphones support technology DSDS (Dual SIM Dual Standby), which means that two cards are in standby mode. However, when you start a conversation on one card, the second one may temporarily lose network or switch to 2G mode. This is not a breakdown, but a feature of the radio module architecture. More advanced technology DSDA (Dual SIM Dual Active) allows you to talk on one line and receive a call on the other, but it is rare.
A common reason for the lack of 4G on the second SIM card is the limited bandwidth of the antennas. Many smartphones have one powerful receiver and one auxiliary one. If the main card operates in LTE Band 3 or Band 7, the second card may be forced to 3G or 2G, since the processor cannot simultaneously handle two data streams in high-speed bands. This is especially true for devices in the mid-range and budget segment.
| Operating mode | Description | Impact on the second SIM |
|---|---|---|
| DSSS | Dual SIM Single Standby | Only one card is active, the second is completely disabled. |
| DSDS | Dual SIM Dual Standby | Both are on the network, but during a call on one, the second becomes unavailable. |
| DSDA | Dual SIM Dual Active | Full operation of both cards at the same time (rare). |
If your operator uses specific frequencies (for example, Band 20 for 4G in rural areas), and the smartphone does not support this range in the second slot, there will be no stable connection. Check the technical specifications of your model on the manufacturer's official website, paying attention to the "Supported bands" section for each slot separately.
Why does the second SIM card drop in 2G?
During a voice call via VoLTE (Voice over LTE), the network may not switch, but if VoLTE is not supported by the operator or phone, the device resets the connection until 3G/2G to provide voice communication, which may temporarily interrupt the registration of the second SIM card in the 4G network.
Problems with IMEI and base station
The most serious software problem is loss or change IMEI (International Mobile Equipment Identity). This is a unique identifier for the equipment. If the phone settings display zeros or dashes in the IMEI field of the second SIM card, the network will not work. This often happens after an unsuccessful flashing of the device, especially on models with processors MediaTek.
You can check the IMEI by typing the universal code *#06# in the "Phone" application. Compare the numbers displayed on the screen with those on the box or under the battery (if it is removable). If the number of the second SIM card is missing or incorrect, recovery will be required through the engineering menu or specialized software, which is best left to specialists.
⚠️ Attention: Independently changing IMEI is illegal in many countries and can lead to the device being blocked by the telecom operator forever. Resort to recovery only if the number is lost as a result of a software failure.
Sometimes the problem lies not in the phone, but in the operator’s profile on the card itself. If the SIM card is very old (produced more than 5-7 years ago), it may not support modern authentication protocols in 4G/5G networks. In this case, even a working phone will not be able to register a second SIM card in the new generation network.
The absence of IMEI or a zero value in the identifier field is a 100% software failure that requires flashing or restoration through service software; physical replacement of parts will not help here.
Conflicts between operators and APN access point settings
Even if the phone sees the network, the Internet may not work due to incorrect settings Access points (APN). This is especially true if you use SIM cards from different operators. For example, settings MTS may conflict with profiles Beelineif they were imported automatically with errors. Go to Settings → Mobile network → Access points (APN) for the second SIM card.
Make sure that the active access point corresponding to your operator is selected. If the list is empty or the settings are confused, create a new access point manually. The input data (name, APN, login, password) can be found on the official website of your operator. After saving, be sure to restart your smartphone.
There is also the problem of the phone being “locked” to a specific operator. If you bought a smartphone abroad or secondhand, it may be blocked to work with only one network. Inserting a second SIM card from another operator into such a device will result in it not working at all. You can check this by inserting a known good SIM card from another provider into the first slot.
- 📱 Check if the phone status is "Sim locked" or asking for an unlock code.
- 🌐 Make sure that a proxy server is not specified in the APN settings if it is not required by the operator.
- 🔄 Try resetting the network settings:
Settings → System → Reset settings → Reset Wi-Fi, mobile data and Bluetooth settings.
⚠️ Attention: Resetting the network settings will delete all saved Wi-Fi passwords and paired Bluetooth devices. Be prepared to enter them again after the reboot.
Radical measures: reset and flashing
If none of the above methods helped, the system may have accumulated critical software junk or a conflict in the radio module configuration files. In this case, a full reset to factory settings helps. Before doing this, be sure to back up all important data, as the process will delete everything from the internal memory.
In extreme cases, when resetting does not help, flashing the device is required. Use only official firmware images from the manufacturer's website. Third-party assemblies (Custom ROM) often contain errors in the modem drivers, which leads to incorrect operation of the second SIM card. When flashing, it is important to select the "Clean All" option to erase old configuration files.
adb reboot bootloaderfastboot flash modem NON-HLOS.bin
fastboot erase modemst1
fastboot erase modemst2
fastboot reboot
This sequence of commands (for advanced users with rights ADB) allows you to overwrite the modem partition and clear its data storage. Be extremely careful: incorrect execution of commands can turn your phone into a brick. If you are not confident in your abilities, it is better to contact an authorized service center.
What is EFS and why should you not touch it?
The EFS section contains unique calibration data for the radio module, including IMEI. Damage to this partition during flashing will lead to the loss of the network forever; restoring it without a factory backup is almost impossible.
Frequently asked questions (FAQ)
Why does the second SIM card only work in 2G, although the phone supports 4G?
Most likely, your smartphone works in DSDS mode with limited antenna capacity. When one SIM card actively uses 4G for the Internet, the second can be software limited to the 2G/3G standard to prevent signal interference. Try turning off the mobile data on the first SIM card.
The phone says “Unregistered on the network” on the second SIM card, what should I do?
Try to manually select the operator in the network settings (Settings → Mobile network → Telecom operators). Disable automatic selection and try connecting to your operator from the list. If it doesn’t help, check if the SIM card is blocked for non-payment.
Can the case affect the operation of the second SIM card?
Yes, some metal or magnetic cases can shield the signal, especially if The antennas of the second SIM card are located in the upper or lower part of the case, which are covered with a cover. Remove the case and check the signal level.
Why did the second SIM card stop seeing the network after updating Android?
The update could reset the APN settings or change the priority of networks. Go to the SIM card settings and reactivate the second slot, and also check the access point settings. In rare cases, new software contains modem driver bugs.
How can I find out if my phone supports two SIM cards in 4G at the same time?
This information can be found in the technical specifications on the manufacturer's website. Look for the mark "Dual 4G VoLTE" or "4G+4G". If there is no such mark, most likely, the second SIM card will work in 3G/2G mode with 4G active on the first one.