In short. The #31# prefix (and its landline equivalent 3651 in France) affects one thing only: caller ID display during a voice call. An SMS doesn't use that mechanism: it travels with a sender header imposed by the network. So typing #31#0612345678 in the Messages app will, at best, send a perfectly identifiable text, and at worst nothing at all.
Information verified on 8 November 2026 on the public pages of Arcep, the CNIL, service-public.fr, Légifrance and cybermalveillance.gouv.fr, as well as the support pages of the French carriers (Orange, SFR, Bouygues Telecom, Free Mobile) and Apple's and Google's documentation. This article provides general information and is no substitute for advice from a legal professional.
What does the #31# prefix actually do?
#31# is a command addressed to the telephone network, not to your phone. Dialled immediately before a number, it asks the network core not to pass your caller identity on to the recipient, for that one call only. This is known as caller ID restriction (in technical jargon: CLIR, Calling Line Identification Restriction).
The key point — and the source of the whole misunderstanding: this service belongs to the world of voice telephony. It is one of the historic "supplementary services" of the switched network, later carried over into GSM and then into voice over IP. It applies to a call attempt. There is no equivalent for a written message.
Tellingly, French carriers describe this prefix in their help sections devoted to calls, not in those about SMS. Orange, SFR, Bouygues Telecom and Free Mobile all offer one-off withholding via #31# and, often, permanent withholding that can be switched on from your online account or by asking customer service — always for voice.
Remember: #31# does not "make you anonymous". It asks the network not to display your number on the other person's phone. Your carrier knows exactly who called, and keeps that record.
Why does this command have no effect on a text message?
Because an SMS doesn't contain a "caller number" in the telephony sense, but a sender field filled in by your carrier's messaging centre (the SMSC). You have no control over it.
Technically, here is what happens when you tap "Send":
- Your phone passes the message to your carrier's messaging centre.
- That centre identifies your line, adds your number as the originating address, along with a timestamp.
- It routes the message to the recipient's messaging centre.
- The recipient's phone displays the number contained in that header.
At no point does the chain consult a "hide my number" setting. There is no checkbox at that level: the sender's identity is part of the message itself, because it also serves to route any reply. A text without an originating address would be a message nobody could answer — and one the network would reject.
It's the same logic as the time of sending, which can't be altered either: some elements are stamped on by the network, not by the sender. We went into detail on this in our piece about the best time to send an anonymous text.
What actually happens if I type #31# before a number in Messages?
Three possible outcomes, depending on the phone and the app used. None of them hides anything.
| What you do | What happens | What the recipient sees |
|---|---|---|
#31#0612345678 in the recipient field | The app rejects the recipient or reports an invalid number | Nothing, the message doesn't go out |
The #31# is kept as is | The network doesn't recognise this number format, send fails | Nothing |
| Special characters are stripped and the number cleaned up | The SMS goes out normally | Your full number, in plain sight |
The third case is the most treacherous: you think you've sent a withheld message, when in fact you've sent a perfectly signed text. If the send fails instead, you're back to the classic causes covered in our article on an anonymous text that won't send.
Another common variant of the misunderstanding: typing #31# in the Phone app to see "if it works", which simply places a withheld call. The recipient sees "Withheld number" or "Unknown" ringing, and you haven't sent any message at all. If you really want to check how your line behaves, the cleanest approach is to use a second prepaid SIM card in a device you have in hand, and observe what actually appears on arrival.
#31#, 3651, 067: which does what?
The French prefixes are easy to mix up. Here's a quick summary, checked against carrier support pages and the numbering plan published by Arcep.
| Prefix / setting | Where | Effect | Affects SMS? |
|---|---|---|---|
#31# + number | Mobile (Android, iPhone) | Hides your number for one call | No |
3651 + number | Landline | Hides your number for one call | No |
*31# + number | Mobile, on some networks | Shows your number even though permanent withholding is on | No |
| "Show My Caller ID" setting | iPhone: Settings → Phone | Permanent withholding on calls | No |
| Carrier-level permanent withholding | Online account or customer service | Permanent withholding on calls | No |
Two practical consequences:
- if you've switched on permanent withholding, your calls go out hidden but your texts keep going out with your number;
- conversely, someone who saw you as a "withheld number" on the phone will get your number in plain sight as soon as you text them.
That's exactly the confusion that catches most people out: you think you have "a hidden line", when all you have are hidden calls. The topic is closely related to calling back a withheld number without revealing your own.
Where to find the caller ID setting on Android and iPhone
For calls only, here are the exact paths as of 8 November 2026:
iPhone (iOS 18 and later)
- Open Settings.
- Tap Phone.
- Tap Show My Caller ID.
- Turn the switch off.
If the entry is missing or greyed out, your carrier doesn't expose it via the SIM card: in that case you'll need to request withholding from the carrier.
Android (Google Phone, current version)
- Open the Phone app.
- Tap the menu (three dots) → Settings.
- Open Calling accounts (the wording depends on the manufacturer skin), then Additional settings.
- Choose Caller ID → Hide number.
The labels vary between manufacturers (Samsung, Xiaomi, Google), but the logic is identical. And in none of these menus will you find an option for messages, for the reason explained above. If you change the setting often, jotting down the exact steps for your model in a pocket notebook saves you hunting through the menus every time — and leaves fewer traces than a note saved on the phone.
So how can you send a message without showing your number?
Since the prefix is useless, you need to change the message's originating address rather than try to hide it. Four families of solutions, each with its limits.
1. An SMS gateway
The message goes out from a provider's infrastructure, with a short code or an alphanumeric sender ID in place of yours. The recipient never sees your line. That's the principle behind sending an anonymous text from this site and the method detailed in our guide to sending without giving your number.
Limit: the anonymity is relative, not absolute. The provider keeps technical logs, and a judicial authority can require their disclosure. To understand exactly what the recipient of such a message sees, read who the sender of an anonymous text is.
2. A secondary number
A second physical SIM, an eSIM, or a service number: your main number stays out of the conversation. It's the most comfortable option when you're expecting a reply. An unlocked dual-SIM phone is enough to keep private life and occasional contacts durably separate, and the steps are laid out in our article on dual SIM as a secondary number.
Limit: in France, identification when opening a mobile line is the rule. A secondary number is not an untraceable number.
3. A pseudonym-based messaging app
Apps that work with a username rather than a phone number let you write without revealing your line. Limit: the recipient has to use the same app, which rules out most situations where the whole point is to reach someone "on their phone".
4. The message that expects no reply
Sometimes the real solution is to accept the asymmetry: a single, self-contained message, with no return channel. We explored this in sending a single anonymous text, without pushing.
What hiding your number will never protect
Even with the right method, three things remain visible or recoverable.
- The timestamp. Date and time travel with the message and aren't negotiable.
- The content. By far the leading cause of being recognised: a choice of words, a turn of phrase, a piece of information few people hold. We devoted a full article to the way of writing that gives you away.
- Technical logs. Carriers retain connection data under the conditions set out in the French Postal and Electronic Communications Code, which can be consulted on Légifrance. They are accessible on judicial request, particularly in investigations into harassment or threats.
In other words: the anonymity of a text message is protection from the recipient, never from the courts. The applicable framework is summed up in our article is it legal to send an anonymous text in France.
And what if you're the one receiving a message from an unknown number?
The confusion around #31# feeds the opposite belief, equally false: that a text from an unknown number must be "hidden" and therefore impossible to trace. In the vast majority of cases, an SMS arrives with a very real originating address: a mobile number, a short code, or a sender name.
Three useful habits:
- Don't call back an unknown number out of reflex. "Ping call" campaigns rely on exactly that gesture, as the French regional press pointed out in late 2025.
- Keep the message as it is, both as a screenshot and in its original form: that's the basis of a case file. The method is described in how to build solid evidence.
- Report abusive messages to 33700, the French national scheme run by the AF2M, and serious incidents via cybermalveillance.gouv.fr or by filing a complaint with the police or gendarmerie.
To filter withheld calls without missing an SMS, the useful settings are gathered in our article on filtering incoming withheld calls. And if you simply want to stop being contacted, the stop being contacted page lists the steps to take.
Finally, a practical note: keeping a legible record of a problematic message is easier on a comfortable screen than by zooming in on a phone. An adjustable phone stand placed next to a computer makes it easier to transcribe an exchange faithfully, and a pack of encrypted USB sticks lets you archive screenshots off the phone, in case the device is lost or reset.
Frequently asked questions
Can you send a text with your number hidden using #31#?
No. #31# only affects caller ID display during a voice call. An SMS always goes out with the originating address added by your carrier's messaging centre.
Is there a code equivalent to #31# for text messages?
No, no French carrier offers a code of that kind, because withholding your number is a voice telephony service with no equivalent in the SMS standard. The codes circulating on forums have no effect whatsoever.
Does my carrier's permanent withholding also hide my texts?
No. Permanent withholding taken out with Orange, SFR, Bouygues Telecom or Free Mobile applies to outgoing calls. Your texts will continue to display your number.
So how do you write without showing your number?
By changing the message's sender: a sending gateway with an alphanumeric sender ID, a secondary number on a second SIM, or a messaging app that works by username. The choice depends mainly on whether you need to receive a reply.
Is a text sent without my number untraceable?
No. The recipient doesn't see your line, but the technical logs of the carrier and the provider exist and can be requisitioned by the courts in the event of an offence, in particular harassment or threats.
Updated 8 November 2026. Setting paths and carrier options change over time: check your phone's menu and your carrier's support page before making any changes.
