Owners of Xiaomi Redmi 9 smartphones, who have looked into the depths of the phone settings, often stumble upon a mysterious item called teletype mode (or TTY). This option is hidden in the menu of special features or settings SIM-It looks like a relic of the past, because today's users are used to video calls and instant messengers. MIUI raises many questions: is the firmware not lost, is this mode necessary for stable communication and what will happen if it is activated?
In fact, Xiaomi Redmi 9 is no exception — it's been in Android for many years, and it was developed in an era where text messages were transmitted exclusively through voice, using audio signals. Today, it's a legacy feature that is disabled by default, but its random activation can change the behavior of your device when incoming calls.
In this article, we will take a closer look at the history of the technology, analyze the need for its use in 2026, and explain how to manage this parameter correctly so as not to disrupt the mobile network, and you will understand whether this option is worth paying attention to at all or you can safely ignore it.
Historical context: from telegraph to smartphone
Teletypewriter (TTY) technology dates back to the first mobile phones, where teletypes were used to transmit printed text over telephone lines, allowing people with hearing or speech impairments to communicate at a distance, and with the advent of cellular communication, the standard has been adapted for mobile devices to allow text to be transmitted over a voice channel.
In the age of button phones, this mode was critical for a specific group of users: it converted text messages into audio signals that were decoded at the other end of the line. Xiaomi Redmi 9, like most modern Android smartphones, retained software support for this protocol to ensure accessibility and compliance with international communication standards.
⚠️ Attention: Activation of the regime TTY On a modern smartphone without the presence of special equipment on the receiving side can lead to the fact that you will not hear the interlocutor during the call.
Today, with 4G LTE video calls and instant messaging, classic teletype seems archaic. However, carriers in some regions still support this standard, so Xiaomi engineers leave this option on the system menu.
Technical features of TTY on Android
The Android-based teletype mode installed on Redmi 9 works by switching the modem to a special data mode. When the function is active, the phone stops using the standard voice codec to transmit sound and begins encoding text into tonal signals. This requires a stable connection and can occupy the voice channel completely.
There are several ways this protocol works that affect how the device handles incoming and outgoing calls, and understanding the differences between them will help avoid confusion when setting up network settings.
- 📞 TTY Off – Standard mode in which the phone works like a normal voice device, transmitting sound in high quality.
- 📝 TTY Full – the device can only send and receive text messages through the voice channel, voice communication is blocked.
- 🗣️ TTY HCO (Hearing Carry Over – allows you to hear the voice of the interlocutor, but respond to him with text (useful for people with speech impairments).
- 👂 TTY VCO (Voice Carry Over – allows you to speak in voice but receive answers in text (for people with hearing impairments).
It is important to note that support for a particular mode depends not only on firmware. MIUI, But also from the capabilities of your mobile operator, if the network doesn't support transmission. TTY-activation of the function will not lead to anything, except potential failures in the display of calls.
Where to find the settings of the teletype on Xiaomi Redmi 9
Location of settings TTY It can vary depending on the version of the Android operating system and shell. MIUI, In older versions of the interface, this item was often hidden in the menu for developers or special features, but in current builds it is moved to the settings. SIM-map.
To find this parameter, you need to follow a sequence of steps. The interface may vary slightly, but the navigation logic remains the same for the entire Xiaomi ecosystem.
☑️ Searching for settings TTY
If you can't find this item in the specified place, try searching by settings. Type the word "teletype" or "TTY" into the search box, and the system automatically prompts you to go to the desired menu, which is the fastest way to get to hidden settings without wandering through the menu for a long time.
In some cases, especially on global firmware versions, this item may be hidden if the region is not covered by the SIM-The card does not imply the use of this technology, however, on Redmi 9 with European or Russian firmware, it is usually available by default.
Do I need to turn on the teletype mode to the ordinary user?
The short answer is no. For 99% of users, turning on TTY on the Xiaomi Redmi 9 is not only useless, it can be uncomfortable, and unless you have a specific medical condition or need to communicate via teletype devices, you don't need this feature.
Moreover, random activation TTY This can cause the phone to not beep or vibrate in its normal mode when it's calling, but to try to connect in text mode. TTY, You may hear strange sounds or not hear them at all.
What happens if you turn on TTY Full?
Modern alternatives such as video calls, voice messaging and voice-to-text relay functions built into Android work much more efficiently and do not require switching modems.
TTY’s impact on communication quality and battery
While teletype mode is designed to save resources in poor communication, its misuse on modern 4G networks can backfire: TTY data transfer requires constant support for a voice channel, even if you are just sending a short message.
This could lead to the following consequences for the Xiaomi Redmi 9 owner:
| Parameter | Standard mode (Off) | TTY mode (Full/HCO/VCO) |
|---|---|---|
| Voice quality | High (HD Voice) | Missing or distorted |
| Battery consumption | Optimal. | Increased (constant search for TTY network) |
| Connection speed | Instant. | Delay in handshake protocol |
| Compatibility | All operators | Only supporting TTYs |
Continuous Network Survey for Availability TTY-The battery of your smartphone will discharge faster than usual, especially in areas of uncertain reception of the signal.
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If you notice that the phone is rapidly discharged in standby mode, check if the teletype mode or other experimental network functions are accidentally activated.
Solving Sound and Calling Problems
Often users search for information about teletype when faced with sound problems: the speaker is missing, the interlocutor is not heard or the call is immediately reset. 90% of the time the problem lies in the downed TTY settings, which may have changed after the system update or reset.
If your Xiaomi Redmi 9 stopped calling normally, follow these steps: Make sure the menu described above selects TTY Off or Off, which will return the modem to standard mode.
⚠️ Warning: If the sound problem persists after switching mode, don’t rush to carry your phone for repair. Try restarting your device or resetting your network settings (not data!) to update your carrier’s connection profile.
Also worth checking is whether the feature "mono-audio" or other availability settings that may conflict with the speaker, a comprehensive check of all audio parameters will help to localize the malfunction.
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Returning TTY settings to off position solves most of the problems with no sound when calling on Xiaomi Redmi 9.
Comparison with modern accessibility technologies
While TTY remains on the compatibility menu, Xiaomi and Google developers are introducing much more advanced tools.Redmi 9 features that make communication accessible to people with disabilities without the need for outdated protocols are available.
- 🎙️ Live Transcribe – instantly translates the conversation interlocutor’s speech into text on the screen in real time.
- 🔊 Sound Amplifier – amplifies quiet sounds and filters background noise through headphones.
- 📹 Video calls – allow you to use sign language through a front camera in high resolution.
These technologies use high-speed internet and powerful smartphone processors, making them much more efficient than the old teletype, and TTY is only valid for very specific corporate or government structures that still use legacy equipment.
So, the owner of Xiaomi Redmi 9 just needs to know where this setting is, so that if necessary, to turn it off, and in everyday life, this setting should remain inactive, ensuring maximum communication quality and autonomy of the device.