Sophie
10 November 2026 · 11 min read
You write to a cousin in Morocco, a colleague in Mexico or a friend in Vietnam. The number has been in your address book for years, in the correct international format, and it used to work. Today, every text fails — or worse, goes out with no error and never arrives. Countries regularly reorganise their numbering plans: a digit added in front of mobile numbers, a carrier prefix replaced, a number length going from 8 to 9 digits. Overnight, a perfectly valid number becomes non-existent, and your phone has no way of telling you.
Updated 10 November 2026.
Can a country really change all its mobile numbers?
Yes, and it happens more often than you would think. Each state manages its own national numbering plan within the framework of Recommendation E.164 of the International Telecommunication Union (ITU). When it runs short of numbers, it creates more by lengthening existing numbers or opening up new prefixes.
The mechanisms are always the same, whatever the country:
- Adding a digit. A country inserts a digit (often a 6, a 7 or a 9) right after the country code for all mobile numbers. An 8-digit number becomes a 9-digit number.
- Opening new prefixes. The country allocates new ranges to a carrier that is running out. Older numbers remain valid, but some are sometimes migrated.
- Regional overhaul. Geographic prefixes are redistributed, which mainly affects landlines but can spill over onto mobiles.
- Carrier takeover. When a carrier disappears, its prefixes are sometimes reassigned and its subscribers renumbered.
France experiences the same phenomenon, in its mild version: the Arcep (French electronic communications regulator) opened new 06 and 07 ranges in 2026 because the old ones were running out. In France, existing numbers were left untouched. Other countries go for the radical option: lengthen everything.
A numbering plan is a finite public resource. When it runs out, it is the user who absorbs the change — and the address books of their correspondents abroad that age badly.
How can you tell whether the problem is an outdated number?
The most reliable sign is the combination of three elements: the failure is permanent, it affects a single contact (or every contact in the same country), and the number is shorter than the ones you see in circulation for that country today.
Here is the sorting grid to apply before drawing conclusions:
| Symptom | Likely cause | What it is not |
|---|---|---|
| Immediate failure, "not sent" message as soon as you send | Invalid number: length or prefix rejected by the network | A network problem |
| Message shows "sent" but never read, for months | Number technically valid but reassigned to nobody, or to someone else | A switched-off phone |
| Failure with every one of your contacts in the same country | Recent national renumbering | A block by your carrier |
| Failure with a single contact, the others go through | Number changed individually, or number reassigned | A national renumbering |
| Calls go through, texts do not | SMS routing problem, not a numbering problem | An outdated number |
That last case is the classic trap: if your calls connect and only your texts fail, the number is fine and the cause lies elsewhere. You then need to look at filtering on the receiving network or at the sending format, as detailed in our article on texts you receive but cannot send from abroad.
The digit-count test
This is the quickest check and it costs nothing. Count the digits of your saved number, after the country code, and compare it with a recent number from the same country — a hotel, a restaurant or a government office found on an official site in that country.
- If your number has one digit fewer, you very probably have a number from before a renumbering.
- If the length is identical but the first digit after the country code differs from every current mobile number, the prefix has most likely been withdrawn from service.
- If everything matches, the number is plausible: look elsewhere.
The reference list is published by the ITU in its bulletin of national numbering plans, updated country by country. It is the only authoritative source; general-purpose dialling-code websites often copy out stale data.
Where can you find the correct format of a foreign number in 2026?
In decreasing order of reliability: the website of the country's regulator, the "contact" page of a large local company, then ITU documents. Forums and reverse directories come a very distant last.
Three sources worth knowing:
- The ITU Operational Bulletin (itu.int) publishes the national numbering plans declared by each administration, with the expected number length and the ranges allocated to each carrier.
- The national regulator (the local equivalent of Arcep (French electronic communications regulator)) announces renumbering several months in advance and generally maintains a page dedicated to the numbering plan.
- A real, recent number from the country: a bank, an airline, a tourist office. Not an official source, but an up-to-date sample.
For frequent travellers, a spiral-bound paper address book remains the dumbest and most effective safety net: it survives phone theft, a dead battery and failed syncs, and you can note down the date on which the number was verified.
Why doesn't my carrier warn me?
Because it does not have the information and is not meant to. Your home carrier routes the text to the destination country's network; it is that network which finds that the number no longer exists. The error code that comes back, when it does, is generic.
Texting works in "store and forward" mode: your carrier's message centre (SMSC) takes the message in hand, then tries to deliver it. Three outcomes are possible:
- Immediate rejection. The destination network replies that the number is unknown. You see a red cross, sometimes a terse system message. That is the best case: you have been warned.
- Silence. The message is accepted by the receiving network then discarded, with no feedback. Your phone shows "sent" and you wait for a reply that will never come.
- Delivery to a stranger. If the number has been reassigned to a new subscriber, your message reaches someone else. Nothing tells you so.
This third case is the most unpleasant, and it explains why delivery receipts protect you from nothing: a receipt confirms that a phone received the message, not that it was the right phone. We cover this discrepancy in why delivery receipts lie when you text abroad.
Errors that look like a renumbering but are not
Before you go and rewrite your whole address book, rule out the far more common causes:
- A number mis-entered in the first place, especially with the national leading zero kept after the country code. That is error number one and it is covered in our guide to the international format of a foreign number.
- Number portability: your contact kept their number while switching carriers. The number is valid, but routing can be slower.
- A virtual or VoIP number that does not accept incoming texts.
- A dialling code shared between several countries, which sends your message to the wrong place while remaining technically valid.
How can you bring your address book up to date without redoing it all?
By working country by country rather than contact by contact, and starting with the ones you have not reached in over a year. A contact you exchange messages with every week necessarily has an up-to-date number; the risk is concentrated in dormant numbers.
A five-step method:
- List your foreign contacts by dialling code. On Android, Google Contacts allows a CSV export; on iPhone, the Contacts app lets you share a card, and iCloud.com exports vCards. Sorting by dialling code makes the groups appear.
- Spot the shortest numbers in each group. Within the same country, two different lengths almost always signal an old format.
- Check the expected length via the country's regulator or the ITU bulletin.
- Correct to strict international format:
+then the country code, then the national number without its leading zero. Never00, never non-breaking spaces pasted from a website. - Test through an alternative channel. An internet messaging app (WhatsApp, Signal) immediately shows whether the number matches an active account: that is a strong clue, not proof.
For bulk corrections, working on a computer with a real keyboard makes all the difference: a hundred cards to rework with your thumbs on a 6-inch screen ends in typos. A compact Bluetooth keyboard is enough to make the exercise bearable, and a tilting phone stand saves you an hour with your neck bent.
What if you have no way of finding the right number?
The useful instinct is to go through a channel that does not depend on the number:
- a known email address;
- an account on a social network where your contact is active;
- a mutual acquaintance whose number does work;
- the postal address, which does not change when a regulator adds a digit.
If your correspondent is reachable through an internet messaging app but not by text, the number may have been kept as a mere account identifier while the line itself was cancelled. That is exactly the situation described in your contact only has WhatsApp: will a text arrive?.
Will renumbering ever stop?
No, and the pressure is growing. Demand for numbers no longer comes only from human subscribers: connected devices, industrial SIM cards and backup lines consume entire ranges. Countries with strong population growth or high mobile penetration are the first to be affected.
Two developments ease the problem without solving it:
- eSIM, which makes changing carrier easy but not changing number: if your contact takes out a local line, they get a new number, full stop. The activation and transfer mechanisms are described in eSIM in France in 2026: how it works.
- RCS, which is still tied to the phone number and therefore suffers exactly the same breakages as SMS.
In other words, the phone number remains the central identifier of mobile messaging, and it remains an identifier that governments can change. The only protection is to date your contacts: a number verified in 2019 does not have the same value as one verified last month.
Key takeaways
| Situation | Immediate action |
|---|---|
| Permanent failure in a single country | Check the expected number length with the local regulator |
| Failure with a single contact | Test another channel, ask for confirmation of the number |
| "Sent" with no reply for months | Assume reassignment: never send sensitive information there |
| Calls OK, texts failing | The number is fine: look at filtering or at the message format |
One last, often overlooked point: until you have confirmed that a foreign number is current, treat it as public. A reassigned number sends your messages — codes, addresses, family information — to a stranger who never asked for them. For a one-off message to a number you have doubts about, our text-sending service and the site FAQ explain what you can check before you write, and what you cannot.
Frequently asked questions
Can a foreign number become valid again? Yes, if it is reassigned to a new subscriber. The number becomes "reachable" again even though your contact is no longer on it. That is why a message that goes out without an error is no proof that it reached the right recipient.
How do I know how many digits a mobile number should have in a given country? By consulting the numbering plan published by the ITU or by the country's national regulator. Expected lengths are listed range by range, with the update date.
Can my phone automatically correct an old number?
No. Android and iOS can reformat a number (add a +, remove a zero) according to the SIM card's country, but no system knows the history of foreign renumbering. The correction is manual.
Should I keep the old number in the contact's card? It is useful for traceability, provided you clearly mark it as obsolete and move it out of the main field, so that an app does not reuse it by default.
Are internet messaging apps immune? No. WhatsApp, Signal and Telegram identify accounts by phone number. When the number changes or is reassigned, the account follows the same logic: it may disappear, or end up linked to a different person.
