By the time you're reading this, the 6300 4G has been well past its lifespan on the shelves, and HMD no longer sell the phone. If you were to buy it right now, you will most likely buy an used or refurbished version.
However, when you do a quick search on eBay, Shopee and other online shopping sites across North America, Europe and Southeast Asia, you may see hundreds of listings of used 6300 4G in various conditions. Although the listings seem to look identical, be warned that many of those are NOT genuine but are knock-offs with terrible build quality and user experience.
Brand-new KaiOS phones, even when off the shelves, don't cost less than 2/3 of their retail prices.
HMD has never sold 2G-only version of any of its KaiOS devices in its official capacity. All KaiOS phones from HMD are well-equipped with 4G LTE, Wi-Fi and Bluetooth. KaiOS devices are required to have 3G at minimum.
If the seller only uploaded generic photos showing the phone's exterior, ask for some additional photos of the box it came in. A genuine phone's box would show all its features on the fine print, as well as an information sticker indicating its model number (which matches the device specifications below) and targeted regional market.
Note: Check the model number on the packaging box with that on another information sticker under the battery, and when dialing *#0000# in the operating system!
Look for signs of the phone running KaiOS and not MRE: KaiOS uses the distinct Open Sans UI font and vibrant, properly aligned UI elements. KaiStore and related services should be available at all time. It does NOT run Opera Mini 4.4 or other Java/MRE apps.
Photos provided by thurmendes on r/KaiOS Discord server in June 2023 and various Reddit posts (second, third, fourth).

Kosher is a category of devices whose software and sometimes hardware are heavily modified to limit access, or remove content and features deemed distracting or against religious values, while keeping other features and the user interface almost identical to the original. There are companies specialising in making Kosher phones, and you can find Kosher phone listings on eBay being advertised as productivity and focus improvement tools.
Because the build and UI remain identical, it's difficult to tell Kosher phones and genuine ones apart. However, there are a few generic ways to differentiate them:
*#*#debug#*#* triggers nothing; cannot boot into Recovery mode; getting access to EDL mode variesKosher is indeed a great way to make your phone truly basic, but HMD already offers more basic phones with 4G in their feature phone lineup, so the choice is yours.
Photo provided by nuxx on r/KaiOS Discord server in October 2021.

North America (US, TA-1324) and Mainland China (CN, TA-1287) versions are customised differently from other 6300 4Gs to comply with local regulations.
TA-1324, approved by the three major US carriers, Verizon, AT&T and T-Mobile, only works on LTE bands 2, 4, 5, 12, 17, 66 and 71. Its modem doesn’t cover global LTE bands (e.g. band 1, 3 and 7), so you may have trouble making or receiving calls and texts overseas. Even then, it lacks band 13, which Verizon primarily uses for VoLTE and coverage in rural area. Other models are also barely compatible with US networks, with only band 5 shared across all; except TA-1307 which also shares LTE band 2 and 4.
On the TA-1324, you can only select English (US), español (US), Français (CA) and Português (BR) as display languages. On the TA-1287, you cannot set Google as a search engine (Baidu is set as default). WhatsApp, Facebook, YouTube, Google Maps and Google are not pre-installed, though you can get the former two from KaiStore.
Although you can still install third-party apps on TA-1324 with ADB and WebIDE as with the other versions, rooting is currently not possible as it is signed with a different PK_HASH signature for handshake in EDL mode. This requires a separate programmer which we don't have in archive. Without root access, you can still use AppBuster to hide unwanted apps from the app list instead.
ssp.kaiads.com in your Wi-Fi routers' blacklist or the system's hosts file. Note that this might prevent you from installing certain apps from KaiStore like WhatsApp.accessibility.screenreader Device Settings boolean flag. This feature might not work well with some third-party apps which have unlabelled buttons.keyboard.gaiamobile.org being too short. [BananaHackers' guide on fixing the keypad speed] may helpkeypad.js in keyboard.gaiamobile.org, set this.isT9Enabled=true and priotise T9 in the typing mode list (kudos to [mrkisl in r/KaiOS Discord server])tip: Save these codes as contacts or as Speed Dial entries for quick use later. Dial and press Call to activate a code's function when it shows in the suggestion list.
*#*#33284#*#* (*#*#debug#*#*): Toggle debugging mode, allowing the phone to be debugged over ADB and DevTools (see ADB and WebIDE). A bug icon will appear in the status bar letting you know debugging mode is on. This can also be turned on under Settings, Device, Developer, Debugger, ADB and DevTools.*#06#: Display the 15-digit International Mobile Equipment Identity numbers or IMEI(s) to uniquely identify a cell phone on GSM networks. Do not modify, delete or show these numbers to anyone else: invalid or duplicated IMEI(s) will prevent you from receiving cellular signals or even get you into legal trouble.*#0606# (TA-1324 only): Display the 14-digit Mobile Equipment Identifier numbers or MEID(s) to uniquely identify a cell phone on CDMA networks. Note that this only applies to US models; on international models the MEIDs would be invalid (all zeroes) and thus this secret code does nothing.*#0000#: Display device information, including firmware version, build date, model number, regional variant and CUID.*#33# (call): Check the Call barring service status from carrier, which is used to block or whitelist calls, whether incoming or outgoing, domestic or international. Requires a 4-digit passcode. To toggle this feature, go to Settings, Network & Connectivity, Calling, Call barring.*#43# (call): Check the Call waiting service status from carrier. To toggle this feature, go to Settings, Network & Connectivity, Calling, Call waiting.*#*#372733#*#* (*#*#draped#*#*): Open KaiOS MMI Test, an internal tool used by OEMs to test the hardware components of a KaiOS device. Tests are done automatically or manually, and include LCD backlight, T9 keyboard, camera, LED flash, RTC, speaker, microphone, vibrator, 3.5mm audio jack, SIM trays, Wi-Fi, Bluetooth, NFC, microSD and microUSB slots.
*#07#: Check the ro.sar.enabled boolean property, if true check the current SAR level and display SAR-related health and safety information.*#1219#: Clear all userspace customizations, presumably for in-store display.*#091# (on)/*#092# (off): Toggle auto-answering on incoming call. You can also turn this on under Device Settings in WebIDE.*#2886# (*#auto#): Should also open KaiOS MMI Test interface.*#8378269# (*#testbox#) OR *#*#2637643#*#* (*#*#android#*#*): Open Testbox engineering menu to test hardware components — can be manually opened using Luxferre's CrossTweak.###2324# (###adbg#): Open a menu which allows toggling Qualcomm diagnostic mode for fixing null/invalid IMEI or baseband via QPST.*#*#212018#*#*: Toggle privileged access (including root ADB shell) to the phone.*#7223# (*#race#): Display internal firmware build and boot image versions.*#*#0574#*#* (*#*#0lri#*#*): Open LogManager utility which allows you to fully enable ADB and DevTools on Spreadtrum devices.*#573564# (*#jrdlog#): Open T2M Log (jrdlog), a stripped-down LogManager interface.*#1314#: Switch the auto.send.crash.sms property, whose purpose is still unknown.Exit these modes by removing and re-inserting the battery, or type and execute the commands provided for each mode.
Press and hold the Power button on the top of the phone and the * key while the phone is powered off, or type and execute adb reboot recovery while the phone is in debugging mode and connected to a computer. Use D-Pad Up and Down keys to move between options, and press the Power button on the top (not the center OK key) to select.
Default recovery interface only allows you to factory reset the phone and start fresh by wiping /userdata and /cache, view boot and kernel logs should the operating system fails to start, and install patches with adb sideload or from SD card. Using EDL mode, you may be able to replace the default recovery interface with a custom one for more features, such as ADB support.
hint: /recovery partition has the same 32.0 MB (32,768 KB) size as /boot, meaning that you can replace /recovery with a copy of /boot for booting into KaiOS, and reserve /boot for i.e. installing other operating systems such as postmarketOS.
fastbootd is a component of the Android bootloader. It allows you to write system partitions in situations where your phone cannot boot normally, or execute manufacturer-specific operations with fastboot oem commands, e.g. unlocking the bootloader to flash custom ROMs.
On the 6300 4G, it is only accessible and will automatically kick in when both /boot and /recovery are corrupted; to manually activate this mode, use dd to wipe both partitions with zeroes. In this mode, the phone only shows a still of the boot splash — the white Nokia logo on a black background.
From here, to interact with the interface, you need to connect your phone with an USB cable, and use the fastboot CLI tool on your computer:
fastboot should be included in the android-tools package, which you can install from Homebrew or from your operating system's package manager;fastboot from the Android SDK Platform Tools package; you may also need to download and install Google's generic driver for your computer to detect your phone in Fastboot mode (right-click the android_winusb.inf file and click Install; requires admin rights).
devmgmt.msc) and look for an Android device with an exclamation mark, right click and select Install Driver..., Browse my computer for drivers, Let me pick from a list of device drivers on my computer, Have Disk... and select the INF file.Android Bootloader Interface.Once done, you can exit Fastboot mode with fastboot reboot.
For a list of commands commonly found in the Fastboot interface, see the Android/Fastboot entry on Gentoo Linux Wiki. Note that not all commands can be used on the 6300 4G.
Press and hold the Power button on the top of the phone and both the * and # keys until the phone vibrates, or type and execute adb reboot edl while the phone is in debugging mode and connected to a computer. Boots into a black screen.
In this mode, you can read and overwrite the partition table at the manufacturer level, over Qualcomm's proprietary Sahara or Firehose (QD-9008) diagnostic protocol. Interacting with the phone requires a digitally-signed MBN/ELF programmer file specifically made for the device, and a middle-man program such as QFIL or edl.py.
Instruction on setting up and interacting with the phone filesystem via EDL mode can be found on its separate Wiki entry.
An EDL programmer for the non-US variants of 6300 4G (other than TA-1324) can be found on BananaHackers' EDL archive website with hardware ID 0x009600e100420029. TA-1324 variant has been signed with a different PK_HASH and needs a different firehose loader which we currently don't have in archive.
As discovered by atipls on Discord and @Llixuma, on the mainboard of the 6300 4G, there are 3 UART testing points: TX, RX and GND just above the SIM2 slot. Shorting TX at 1.8V and GND takes you to Fastboot mode and the Linux terminal interface.

By default, KaiOS's Linux kernel disables the UART testpoints; logs from UART testpoints will stop once the kernel kicks in. To read the full output from UART, compile the Linux kernel from HMD's open-sourced kernel release with LEO_defconfig flag (not LEO_defconfig-perf).
You can also force reboot the phone at any time by pressing and holding the top Power button and the # key.
TODO