10 Common SIM Errors in Japan and How to Fix Them Yourself in 5 Minutes

Losing internet connection on your phone in a foreign country always creates an undescribably panicked feeling — unable to call anyone, look up anything, or know who to ask in Vietnamese. Stay calm: 90% of SIM issues can be fixed by yourself in a few minutes if you know where to check. Here is the list of 10 errors we encounter the most when supporting customers, ordered from most common, with each error including troubleshooting steps in recommended order.

Issue 1: Signal bars displayed but unable to connect to the internet

Common cause: incorrect or missing APN configuration — the "address" for your device to communicate with the carrier network.

1. Reopen the email/instruction sheet included with the SIM and find the APN section (name, username, password).
2. iPhone: Settings → Cellular → Cellular Data Network → enter APN exactly as instructed. Android: Settings → Network → Access Point Names → Add new, fill in, and save.
3. Select the newly created APN and restart your phone.

Tip: capitalization and periods in the APN must be typed 100% accurately — a single wrong character will prevent it from working.

Issue 2: Phone displays "No Service" (圏外 / No Service)

1. Turn on Airplane mode for 10 seconds, then turn it off — forcing the phone to search for signal again.
2. Restart your device.
3. Remove the SIM, gently wipe the chip surface with a dry cloth, and reinsert it properly into the tray.
4. Go to Settings → select network manually: if the list of carriers appears but you still cannot connect, the issue is with your SIM account (not activated/suspended) — contact your provider.
5. Test the SIM in another device (borrow a roommate's): if the SIM works in another phone, your phone has an issue (network locked, hardware fault); if the SIM doesn't work anywhere, the SIM/account has an issue.

Issue 3: Lost internet right after updating the OS

Updating iOS/Android sometimes deletes the APN configuration. Fix: reinstall APN as in Issue 1. This is why you should save a screenshot of the APN settings in your phone album from the first day you receive the SIM.

Issue 4: Data runs out rapidly even though you "didn't use anything"

1. Go to data usage statistics (Settings → Cellular / Data usage) to see which apps are at the top of the list.
2. Classic culprits: backing up photos to the cloud over cellular data, auto-playing videos on Facebook/TikTok, automatic app updates, or forgetting to turn off personal hotspot after sharing.
3. Adjust settings: back up only over WiFi, turn off video autoplay, update apps only over WiFi, and turn on Low Data Mode/Data Saver.

Issue 5: Unable to send/receive verification SMS (OTP)

Remember the rule: data-only SIMs do not have traditional SMS functionality — OTPs for Vietnamese bank accounts must go to a Vietnamese SIM (enable roaming to receive SMS, free incoming SMS on most Vietnamese carriers), while OTPs for Japanese services require a Japanese SIM with a phone number. If you are using a Japanese calling SIM and still cannot receive SMS: check if your inbox is full, spam blocking is overly aggressive, or the SMS Service Center (SMSC) number is incorrect — call your provider to check.

Issue 6: eSIM "disappears" or shows an error after switching phones / factory reset

eSIM is embedded in the phone's chip — factory resetting can erase the eSIM (the phone will ask beforehand, read carefully and do not tap randomly). If accidentally deleted: don't panic, your plan is still active with the carrier. Contact your provider to request a reissued activation QR code and scan it again. When switching to a new phone, follow the eSIM transfer procedure (iPhone supports direct transfer between two devices; otherwise, request a new code).

Issue 7: Unusually slow connection during lunch hours and evenings

This is not exactly an "error" — budget SIMs (MVNO) share bandwidth, so they slow down during peak usage hours (12:00–13:00, 20:00–22:00). How to cope: download heavy content outside these time slots and lower video call quality. If your work requires fast internet at all times, consider upgrading to a more stable plan/carrier — see our comparison article between budget SIMs and major carriers.

Issue 8: Strange charges / higher monthly bill than expected

1. View detailed bills on the carrier's account management page.
2. Three items that often cause surprises: voice call charges (regular calls made outside apps charged per minute), add-on services pre-installed with "first month free" that you forgot to cancel, and carrier billing (app/game purchases added to your SIM bill).
3. Cancel unnecessary services right on your account page; turn off carrier billing if you don't use it.

Issue 9: Phone brought from Vietnam does not accept Japanese SIM

• Carrier-locked phone: phones purchased on installment contracts in Vietnam may be locked against foreign carriers. Check: on iPhone go to Settings → General → About → Carrier Lock section must state "No SIM restrictions". If locked, contact your Vietnamese carrier to unlock it (usually granted if installments are fully paid).
• Missing frequency bands: very old phones or specific regional models may lack Japanese network bands — resulting in weak or no signal in some areas. Most phones from 2018 onwards are fine.

Issue 10: Lost phone / lost SIM

1. Lock SIM immediately: message your provider's support channel to request suspension — preventing unauthorized use of your number.
2. Use "Find My iPhone"/"Find My Device" from another device to locate and lock your phone remotely.
3. Report to police (nearest police box — 交番): lost items in Japan are returned to owners surprisingly often; you will need the police report receipt if filing for insurance later.
4. Request SIM/eSIM reissuance to your address — a small fee may apply depending on the vendor.

Quick Lookup Table: Symptom → Section to Read

Conclusion

Before taking your phone to a store or buying a new SIM, run through three quick reflexes: toggle Airplane mode on/off → restart device → check APN settings — this trio resolves over half of all cases. Still stuck? Take a screenshot of Settings → Cellular (or SIM Management) and send it to HD TELECOM support team along with a one-sentence description — a real human speaking Vietnamese will guide you step-by-step until your phone connects to the internet again.