Selecting and setting up a SIM card for calls on Android

A modern Android smartphone is a complex device that requires the correct configuration for stable operation in cellular networks. An incorrectly selected or incorrectly installed SIM card can cause constant connection failures, loss of signal in buildings, or the inability to use modern voice communication functions. Choosing the right media is the first step to quality communication.

In this article we will examine in detail the physical differences between card formats, the technical nuances of working with 4G and 5G networks, as well as the software settings necessary to ensure priority for voice calls. You will learn how to avoid common mistakes when installing a module in a slot and why old cards may not work in new devices.

Physical formats and tray compatibility

The first thing a user encounters when buying a new SIM card or replacing a device is the physical size of the chip. The industry has come a long way from bulky credit-card-sized cards to microscopic modules. Three main standards are relevant for Android smartphones today, and their incompatibility can lead to the card simply not entering the tray or not having contact with the pins of the device.

The most common standard is Nano-SIM. This format measures 12.3 x 8.8 mm and is 0.67 mm thick. The vast majority of modern smartphones released over the past 7–8 years use this type. If you purchase a new card from an operator, you will most likely be given a universal “3-in-1” version, from which you can squeeze out the desired size along the perforations.

However, there are devices that support the technology eSIM (embedded SIM). This is a virtual module built directly into the phone's motherboard. To activate it, you do not need to physically replace the card; you just need to scan the QR code provided by the operator. This is ideal for those who travel frequently or want to use two numbers on one device without having to find a second slot.

⚠️ Warning: Never try to cut an old SIM card by hand with scissors to fit it to the Nano size. You can damage the contact pad of the chip or disrupt the geometry, which will lead to the card getting stuck in the tray and breaking the reader.

It is also important to consider the thickness of the card. Older Mini-SIM and Micro-SIM formats often have a thicker plastic backing. Trying to insert such a card into the Nano-SIM slot using an adapter may result in jamming of the ejection mechanism.

Technical characteristics and network support

The choice of SIM card depends not only on its size, but also on the generation of technology embedded in the chip. Telecom operators are constantly updating their fleet of cards to ensure operation in fourth and fifth generation networks. Old cards released more than 10 years ago may not support the necessary encryption protocols or frequency bands.

The key technology for quality calls today is VoLTE (Voice over LTE). It allows voice traffic to be transmitted over a 4G network, providing high-definition audio (HD Voice) and fast connections. For this feature to work, the SIM card must be updated to the USIM standard. Without this, the phone will automatically switch to 3G or 2G mode during a call, which degrades call quality.

It is also worth paying attention to the support for frequency ranges. Different operators use different frequencies to cover an area. If your SIM card is too old, it may not work correctly in the Band 3, Band 7 or Band 20 bands, which are actively used in cities and beyond to ensure stable coverage.

• 📶 USIM —a modern card standard required to work in 4G/LTE networks and support VoLTE.
• 🔒 ISIM —an application on the card required for IMS services, including video calls via operator's network.
• 🔄 OTA update —the operator's ability to remotely update the card profile without physically replacing it in the salon.

If your smartphone displays the 4G icon, but when you try to make a call, the network drops to 3G (H+), this is a sure sign that the SIM card does not support VoLTE or the service is not activated on the side operator. In this case, replacing the media with a new one usually solves the problem instantly.

Installation process and identification of slots

Installing a SIM card into an Android smartphone requires care and understanding of the design of the tray. Most modern devices have a combo slot, where the second slot can be occupied by either a second SIM card or a microSD memory card. Incorrect installation may result in the phone not seeing the network at all.

To remove the tray, use a special paperclip, which is included in the package. You need to find a small hole next to the tray at the end of the device and press it until it clicks. After this, the mechanism will push out the platform. It is important not to confuse this hole with the noise-canceling microphone, the pressure on which can damage the device.

When installing the card, you must strictly observe the orientation. There is usually a cutout (key) on the card itself and inside the tray that matches the shape of the plastic base. If the card does not lie perfectly flat, do not press on it. Turn it over or turn it 90 degrees.

Correct orientation: The beveled corner of the card must coincide with the beveled corner in the tray.

In dual-SIM devices there is often a difference between the slots. Slot 1 is usually the main slot and supports all features including 4G/5G for data and voice calls. Slot 2 in some budget models can only operate in 2G/3G mode. This is critical to consider when choosing a slot for the main SIM card with which you plan to make calls.

Setting priorities and network modes in Android

After physically installing the card, you need to correctly configure the software part of the Android operating system. By default, the system can select the network automatically, but for stable calls, manual selection is sometimes required. This is especially true in areas of poor reception or when using cards from different operators.

To access the settings, go to the menu Settings → Connections → SIM card manager (the path may differ slightly depending on the manufacturer’s shell, for example, MIUI or OneUI). Here you can set the default card for voice calls. If you have two active cards, the system will ask each call which one to use, unless a default rule is set.

An important parameter is choosing the preferred network type. For the best call quality, it is recommended to set the value to 5G/4G/3G/2G (auto). Forced restriction to only the 4G network can lead to problems if there is LTE coverage in a specific location, but the VoLTE service is not configured correctly.

Setting parameter	Recommended value	Impact on calls
Network type	5G/4G/3G/2G (Auto)	Provides the best available signal
Voice calls	SIM 1 (Primary)	Fixes the main number for outgoing
VoLTE	Enabled	Allows calls via 4G without dropping to 3G
Data roaming	Disabled (at home)	Prevents charges for border

In some cases, operators hide the VoLTE enable menu. If you are sure that the card and phone support the function, but there is no switch, you can try entering the engineering code through the app Phone. For example, the code ##4636## opens the testing menu, where you can force VoLTE Provisioned to be turned on.

⚠️ Attention: Settings in the engineering menu may change the operation of the radio module. Do not change settings that you do not understand, as this may lead to a complete loss of connection.

What to do if the VoLTE button is gray and inactive?

If the VoLTE switch is inactive, this may mean that the operator has not certified this phone model to work on its network. Try updating the device firmware or changing the SIM card to a newer one.

Solving problems with connection detection and quality

Even with proper installation and configuration, situations may arise when Android does not see the SIM card or the quality of calls remains unsatisfactory. Most often, the problem lies in oxidation of the contacts or a software failure of the radio module, and not in a physical breakdown of the device.

The first step in diagnosis is cleaning the contacts. Take the SIM card and gently wipe the golden contact pad with a soft cloth lightly dampened with alcohol. Do not use abrasives or water. Then blow out the slot in the phone with compressed air to remove dust.

If cleaning does not help, try moving the card to another slot or to another phone. This will localize the problem: if the card does not work everywhere, it means it is damaged or blocked by the operator. If everything works in another phone, the problem is in the radio module or firmware of your Android smartphone.

• 🔄 Reboot — simply turning off and turning on the phone resets network registration errors.
• ✈️ Airplane mode — turn on airplane mode for 10 seconds and turn it off to force reconnection to tower.
• 🗑️ Resetting network settings is a feature in Android that deletes saved Wi-Fi and Bluetooth settings, and also resets the mobile network settings to factory settings.

In rare cases, the problem may be related to IMEI blocking. If the phone was purchased second-hand or brought from abroad, check its status in the regulator’s database. A device blocked by IMEI will not be able to register on the network, regardless of the quality of the SIM card.

Security and protection against fraud

Using a SIM card for calls carries not only technical, but also security risks. Fraudsters often use social engineering techniques to gain access to your number, which is the key to banking applications and social media accounts. Protecting the card itself and the PIN code becomes critical.

Always install PIN code on the SIM card. This will prevent your number from being used if your phone is stolen. The code is requested every time you turn on the device before loading the operating system. Do not use standard combinations like 0000 or 1234, set a unique code and write down the PUK code in a safe place.

Be careful with messages about replacing the SIM card. If you receive an SMS from your operator asking you to confirm your data or follow a link to “activate a new card,” do not do this. Operators never request confidential data via SMS. This is an attempt to steal your number (SIM-swapping).

⚠️ Attention: Never give the PUK code to strangers. Entering an incorrect PIN code three times blocks the card, and entering an incorrect PUK code ten times leads to its complete irreversible blocking and the need for reissue.

It is also recommended to regularly check the list of connected services in the operator’s personal account. Fraudsters can quietly install paid subscriptions or call forwarding to their number in order to intercept your confirmation codes from banks.

Frequently asked questions (FAQ)

Is it possible to use an old SIM card in a new smartphone with 5G support?

Physically - yes, if the dimensions match (Nano-SIM). However, to work in 5G networks and take advantage of all the benefits of VoLTE, it is strongly recommended to replace the card with a new one at the operator’s store. Old chips may not support the necessary encryption algorithms for new networks.

Why does the phone write “No network” or “Emergency calls only”?

This can happen for several reasons: the SIM card is installed incorrectly, the contacts are dirty, the card has expired, or the phone is locked for a specific operator (Lock). Try rebooting the device and checking the network settings.

What is the difference between Dual SIM Dual Standby and Dual SIM Active?

In Dual SIM Dual Standby (DSDS) mode, only one card is active for data or voice transmission at a particular moment, the second is waiting. In Dual SIM Active (DSA) mode, both cards can work simultaneously, allowing you to receive a call on the second line without interrupting the conversation on the first, but this consumes a lot of battery.

How to transfer contacts from an old SIM card to a new phone?

Insert the card into the new phone, open the application Contact, go to settings and select "Import/Export contacts". Select the source "SIM card" and destination "Phone" or your Google account to save in the cloud.

What should I do if I lost the PUK code from the SIM card?

Do not try to guess the code. Find the plastic card holder that originally contained the SIM card - the PUK is usually listed there. If the holder is lost, the only way to find out the code is to contact the operator's communication center with a passport or log into your personal account on the website.