In brief. An anonymous text message is never displayed "with no sender": your phone always shows a label, and it takes one of three forms. A short code of 5 or 6 digits (e.g. 38xxx), a full mobile number starting with 06 or 07 that you do not recognise, or a name in letters ("INFO", "ALERTE"). Each format gives away the sending method and determines what you can do: reply, block, report, or have the author identified through the courts.
Information verified on 17 October 2026 on the public pages of Arcep, the CNIL, AF2M (the 33700 scheme), service-public.fr, Légifrance and cybermalveillance.gouv.fr. This article provides general information and is no substitute for filing a complaint or seeking legal advice.
Why can't a text message arrive "with no sender"?
Because the protocol forbids it. Unlike a phone call, where the network can withhold the caller's number on request (that is the role of the #31# prefix), a text message necessarily carries an originating address field, filled in the moment the message enters the network. The operator's messaging centre, the SMSC, rejects any message whose field is empty.
This is why dialling #31# before a number does not make a text message anonymous: the option acts on voice signalling, not on the header of the text message. We go into this very common confusion in our article on calling back a withheld number without revealing your own.
The practical consequence: the anonymity of a text message never consists in removing the sender. It consists in putting something other than your personal number there. And that "something else" leaves a readable signature.
What are the three possible sender formats?
Three, no more. Here is how to recognise them and what they imply.
| What is displayed | Technical name | Likely sending method | Can you reply? |
|---|---|---|---|
| 5 or 6 digits (e.g. 38xxx) | Short code (MT SMS) | Professional gateway, online service | Sometimes, to the service |
| Unknown 10-digit 06/07 number | French mobile number | Prepaid SIM card, second number, app with a dedicated number | Yes, directly |
| Letters (e.g. "INFO") | Alphanumeric sender name (sender ID) | Bulk messaging platform | No, technically impossible |
| +33 6… or foreign dialling code | International number | Foreign line, gateway outside France | Yes, at international rates |
The five-digit short code: who is behind it?
A short code never belongs to a private individual. It is a numbering resource allocated under the numbering plan managed by Arcep, operated by a carrier or an aggregator that resells it to businesses and messaging services. A message coming from one has therefore passed through a professional infrastructure, with billing, a contract and connection logs.
For the recipient, this is rather good news: anonymity here is relative. The sender is invisible to you, not to the gateway operator. Every send is tied to a customer account, an IP address and a timestamp, retained for periods set out in the regulations on connection data.
Short codes starting with 38 are also reserved by convention for non-premium-rate uses such as notifications. If such a message asks you to call back a premium-rate number or to click a link, treat it as an attempted fraud rather than a personal message.
The unknown mobile number: the most frequent case
This is the classic "a text from a number I don't know" situation. Here the sender has used a real mobile line: a prepaid SIM card bought in a shop, a second number added to their plan, a secondary eSIM, or a messaging app that assigns a dedicated number.
Since eSIM became widespread across the four French operators, this setup has become trivial: a single dual-SIM-capable device carries a main line and a contact line, with no tray to fiddle with and no adapter. For those who keep a phone with a physical SIM, a simple SIM card removal kit saves you from damaging the slot with a paper clip — a trivial detail, but it is the first part anyone loses.
The legal difference matters: a French mobile line is tied to an identified subscriber at the operator, even on prepaid, since activating a prepaid card requires proof of identity to be collected. Anonymity holds only as long as nobody requests that information through legal channels.
The name in letters: the sender you cannot call back
A message signed "INFO", "ALERTE" or with a brand name uses an alphanumeric sender ID. This field accepts letters instead of digits, up to eleven characters. It offers the strongest appearance of authority and, paradoxically, the weakest interactivity: since it corresponds to no line at all, your phone has no number to dial. Any attempt to reply fails silently or stays stuck in the Messages app.
This is precisely the format that the fight against caller identity spoofing has brought under control in recent years. The rules on number display have evolved to prevent the use of labels impersonating a public authority or a bank, a programme of work overseen by Arcep and relayed by service-public.fr. For the reader, the simple rule is this: a sender name is not proof of identity, it is a free-text field filled in by a platform's customer.
How do I read the sender of an anonymous text I have just received?
In three steps, before any emotional reaction.
- Record the exact label. Note, character by character, what appears at the top of the conversation. A
0versus anO, a+33 6versus an06, an extra space: these details matter if you file a complaint. - Check the length. Five or six digits → professional gateway. Ten digits starting with 06 or 07 → mobile line. Letters → bulk messaging platform. That is the whole diagnosis.
- Capture the screen with the time. A dated screenshot, or better still a photo of the screen showing the message timestamp, is worth more than simply copying and pasting the text. Investigators work from the time of arrival as much as from the content.
Two pitfalls to avoid. Do not reply "who are you?" to a short code or a name in letters: all you confirm is that the line is active, which has commercial value for a cold caller. And do not trust sites that promise to "reveal" the owner of a number: no public directory lists mobile subscribers without their consent, and no private service has access to an operator's logs. We explain this in detail in our analysis of whether an anonymous text message is genuinely untraceable.
What if the message is threatening or repeated?
Then the sender becomes identifiable, but not by you. Harassment by electronic means and threats are punishable under the French Criminal Code (notably articles 222-33-2-2 and 222-17), and an investigation can lead to a judicial order served on the operator or the messaging platform to trace the sending account.
The two useful channels:
- 33700, the scheme run by AF2M with the operators, for unwanted commercial or fraudulent messages: you forward the message to 33700, then send the sender's number in a second message.
- Filing a complaint, at a police station, a gendarmerie or online depending on the case, for anything involving harassment, threats or blackmail. cybermalveillance.gouv.fr points you to the right procedure.
In between the two, this site's stop being contacted section sums up the blocking options available on the phone side and the operator side.
On the sender's side: which format should I choose if I want to stay discreet?
The one that matches your actual goal, because the three do not protect the same things.
If you want a reply, the alphanumeric name is ruled out from the start: there is no return path. That leaves the short code with a reply service, or the dedicated mobile number. We compare these options on the page devoted to choosing between an anonymous text message and a disposable number.
If you want nobody to call you back, the name in letters is on the contrary the most airtight — but it is also the most tightly regulated, and for a private individual its use necessarily goes through a professional platform that will ask you for supporting documents.
If you want lasting discretion, a secondary mobile number is the most flexible solution: you give it out, you take it back, you change it. That is the approach described in our guide on how to stop handing out your personal number. If that second number lives in a separate device, a compact power bank keeps you from leaving it switched off at the wrong moment, and a basic keypad phone is more than enough: it carries no account, no cloud backup, no contact syncing.
Useful rule: the further the displayed sender is from a named line, the stronger the anonymity on the recipient's side — and the more it relies on an intermediary who knows everything.
What the sender label never tells you
Three limits to keep in mind, in both directions.
It does not tell you the real country. A French number on display may have been injected from a foreign platform, and a foreign dialling code may belong to someone living two streets away. A sender's location cannot be read from the prefix.
It does not tell you which channel was used. A message arriving via RCS or an encrypted messaging service appears in the same Messages app on many Android phones, sometimes with a discreet mention of the channel. Check that mention before concluding you are dealing with an SMS: the traceability rules are not the same.
It does not tell you the intent. An unknown number is most often a shopkeeper, a delivery driver, a doctor changing lines or a friend travelling. Before assuming harassment, reread the message: the vast majority of anonymous text messages are not malicious, as shown by the situations in which an anonymous text protects privacy.
One last practical habit, for those keeping evidence: a hardback notebook for writing down timestamps by hand remains the most robust method when the phone in question is precisely the one you will have to hand over to investigators.
Frequently asked questions
Can a text message be sent with no sender displayed at all?
No. The originating address field is mandatory in a text message, and the messaging centre rejects any message where it is empty. A so-called "anonymous" text always displays something: a short code, an unknown mobile number or a name in letters.
What does it mean when a text comes from a 5-digit number?
That it was sent via a professional platform using a short code allocated within the numbering plan overseen by Arcep. It is never a private individual sending from their own phone. If it is commercial or fraudulent, report it to 33700.
Why can't I reply to a text signed with a name?
Because an alphanumeric sender name corresponds to no telephone line: there is no number to dial for a reply. Your phone then shows an error or leaves the message pending indefinitely.
Can a website tell me who a number from an anonymous text belongs to?
No. Mobile subscribers do not appear in any public directory without their consent, and no private service has access to operators' logs. Only a judicial order, as part of an investigation, makes it possible to trace the author.
Does the #31# prefix hide my number for a text message?
No. #31# acts on number presentation in voice telephony only. For a text message, you need to go through a different sender: a gateway, a secondary number or a dedicated app.
Updated on 17 October 2026. The rules on number display and the permitted sender formats are evolving: check the information on the official Arcep and service-public.fr pages before taking any steps.
