Xiaomi smartphone owners often face an annoying situation: during a conversation on a mobile network, the device automatically switches to the 2G or 3G standard, completely cutting off access to the network. At this point, notifications from messengers do not come, and the navigator stops building a route, which causes serious discomfort. The problem lies in the peculiarities of the communication modules and the incompatibility of some carrier standards with MIUI software.
The main reason is how voice calls are handled on today’s networks: If LTE voice technology is not activated, the phone has to roll back to older communication protocols that do not support simultaneous voice and data transfer. This is often a surprise to Redmi and Poco users, as it works out of the box in more expensive flagships.
Fortunately, most of the time, the problem is software-solved without having to bring the device into service, so you'll need to check the operator's settings, update the network configurations, and maybe activate hidden engineering menus, and let's go through every aspect of mobile Internet in detail as you talk.
VoLTE Technology: The Basis of a Stable Connection
A key element of the Internet’s operation during a conversation is Voice over LTE (VoLTE), which allows voice traffic to be transmitted as a data stream through a fourth-generation network without requiring switching to outdated 2G or 3G standards. Without the support of the operator and the phone, simultaneous voice and data are impossible.
Many users do not even know that their plan or SIM-The card may not support this protocol by default, and carriers are gradually adopting this technology, but at old rates or using long-term unchanging. SIM-In Xiaomi smartphones, the setting is often hidden or depends on the region of the firmware.
To check if the technology is active, look for the "HD" or "VoLTE" icon in the status bar at the time of the call. If the icon appears, then the voice is transmitted over 4G, and the Internet should work steadily. The absence of a marker indicates that the device fallsback is rolling back on 3G/2G, where parallel work is often blocked.
- 📶 VoLTE – Voice Transmission Technology LTE, Providing high quality sound and Internet.
- 🔄 Fallback: The process of forced network switching to a lower standard (3G/2G) impossible LTE.
- 📱 Dual SIM — Two SIM card mode, which often creates additional data transfer restrictions.
⚠️ Note: Even if the phone menu includes a VoLTE switch, the carrier can block this feature at the network level.
APN settings and operator access points
Often, the cause of failures is incorrect access point settings (APNs), which tell the phone exactly how to connect to the operator's gateway to access the global network, and if the APN profile indicates the wrong type of protocol or no LTE support, the Internet will fall on any voice call.
To correct the situation, you need to manually check the configuration. → SIM-maps and mobile networks → Choose. SIM-map → Access points (APN). You need to make sure that in the field "type" APN» The default,supl or default,supl,dun is written. The absence of the desired parameters can block the data transmission.
In some cases, creating a new access point from scratch helps: copy settings from your operator’s official website or reset to factory settings by clicking the menu button (three dots) and selecting “Reset to default settings.” This action often eliminates software configuration conflicts.
☑️ Checking settings APN
The impact of Dual SIM mode on data transfer
Using two SIM cards is one of the most common causes of communication problems on Xiaomi. Most budget and mid-budget models have a modem that physically can't support full 4G on both cards simultaneously during an active voice call. When you call with SIM 1, the second SIM is often forced to 2G/3G mode or completely shut down.
If the Internet is configured on the card from which the call is made, but VoLTE is not supported, the phone will switch to 3G, and the Internet can be interrupted if the operator does not support SVLTE (Simultaneous Voice and LTE) technology.
To minimize this, try temporarily disabling the second SIM card and checking the Internet, and if the problem is gone, then the modem is limited, in which case it is recommended to assign the main card for calls and the Internet, and use the second card only for incoming ones.
| Mode of work | 4G support for both SIMs | Internet when calling | The need for VoLTE |
|---|---|---|---|
| One SIM | N/D | It works (if there is a VoLTE). | Tall. |
| Dual SIM (Standby) | No (only one active) | Often disappears | Critical. |
| Dual 4G (VoLTE) | Yes (limited) | Stable. | Required. |
| 3G/2G only | No. | It's not working. | Not applicable. |
Why is the second SIM losing its network?
Engineering menu and hidden settings Xiaomi
There are hidden menus in the MIUI shell that allow you to force functions that are hidden from the average user, and one of these tools is the engineering menu, which you can access through a standard caller, which is a powerful tool, but it requires caution.
To enter the modem settings menu, dial ##4636##. If the menu opens, select Phone Information. Here you can find the Enable VoLTE Provisioning switch, which can be activated in the normal settings menu.
However, newer versions of Android and HyperOS often block the code from developers, so you can try to type in code ##86583#, which enforces a VoLTE regional restriction check, and then type in the code, "VoLTE carrier check was disabled."
Codes for verification and activation:
#4636## - Engineering menu (testing)
#86583## - Enable VoLTE region check
#868543## - VoLTE status check⚠️ Warning: Don't change the settings in the engineering menu unless you're sure they're important. Incorrectly changing the parameters of the radio module can lead to the complete disappearance of the network.
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If the code 4636 doesn’t work, try installing the Activity Launcher app from Google Play. Search for the “IMS Service” or “VoLTE” app and try running hidden settings activity.
Resetting network settings and software updates
Accumulated errors in system communications files can also cause connection breaks. Over time, the network services cache gets clogged, and the phone begins to mishandle switching requests between towers. A simple reset of network settings often solves the problem faster than complex manipulations.
To perform the reset, go to Settings → Connection & Sharing → Reset Wi-Fi, mobile networks, and Bluetooth. Please note that this action will remove saved Wi-Fi passwords and associated Bluetooth devices, but will not affect your personal files and photos.
It is also critical to keep up with firmware updates. Xiaomi regularly releases security patches and modem improvements. If you have global firmware installed but live in a region where frequencies differ, the Internet may not work smoothly, and in such cases, flashing to a region that matches your location (e.g. Russia/Europe) helps.
- 🗑️ Network reset – clears the connection cache and resets settings APN factory-to-factory.
- 📡 Firmware – phone software containing modem drivers.
- 🌍 Region – geographical reference of software that affects the list of available frequencies and operators.
Hardware restrictions and operator choice
And the physical layer is also important. Some Redmi and Poco models have modems that don't support all the band bands that carriers in your area use, for example, if a carrier uses 4G on Band 20 and the phone doesn't support it, the phone will drop into 3G when it's away from the tower, where the Internet won't work when it's called.
The most critical reason for the loss of the Internet is the lack of support for the Band 20 (800 MHz) in budget Chinese versions of smartphones when used in rural areas, which provides long-distance coverage and is often the only one available outside the city.
In addition, different operators implement VoLTE support differently, and the Internet can work steadily on one operator, and on another it can be permanently disconnected, because the operator profile (Carrier Config) may be incorrectly defined by the system.
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If the internet is only lost in places or with a specific operator, the problem is most likely in the band coverage or the operator’s settings, not the phone breakdown.