Text blocked by a foreign carrier's spam filter

The number is in the right format, your contact has coverage, your plan includes international texts. You hit send. Nothing. The next day you try again with a shorter message: it arrives instantly. Between your French carrier and your recipient's phone, the receiving network runs every international text through an anti-spam filter called an "SMS firewall". That filter silently deletes messages that look like spam: a clickable link, a sensitive keyword, a one-time code, repeated sending to the same number, an unknown sender. The message isn't late, it was thrown away — and nobody tells you.

Updated 8 November 2026.

Why would a foreign carrier block my personal text?

Because it doesn't know it's personal. The filter doesn't read intent, it applies statistical rules to the content, the sender and the sending rhythm. A legitimate message that ticks the same boxes as a fraudulent campaign meets the same fate.

Mobile carriers worldwide began rolling out this equipment in the mid-2010s for a simple reason: text messaging has become the preferred channel for fraud. The phenomenon is documented in France by the Arcep (the French electronic communications regulator) and by the cybermalveillance.gouv.fr campaigns on SMS phishing. Abroad, regulators have imposed equivalent obligations, often harsher ones: in several countries, an incoming text containing a URL is destroyed outright.

The consequence is counter-intuitive. Your message "Sending you the booking link: booking.com/…" won't be read as practical information between two friends, but as yet another phishing attempt. On the sender's side, no alert: the French network did hand the message over to the foreign network, and its job ends there.

A text deleted by a receiving filter is often counted as "delivered" by your carrier: handover happened between networks, not between phones.

What exactly is an "SMS firewall"?

It's a piece of equipment placed at the entrance of your recipient's network, inspecting texts coming from outside before routing them to the subscriber. It decides whether to let them through, delay them, or delete them.

Technically, it sits on the SS7 or Diameter interconnection links between carriers. The industry's major players — Infobip, Sinch, Twilio, Vodafone Carrier Services, Orange Wholesale — publicly document how it works on their technical pages aimed at businesses. Three families of checks stack up:

Content checking is the one that concerns you. It relies on pattern lists: digit sequences resembling a verification code, banking brand names, words linked to money or parcels, and above all URLs. These lists are maintained by the carrier and its suppliers, and they change every week depending on the fraud campaigns under way.

Which messages travel badly abroad?

Messages containing a link, a code, a sum of money, or sent in bursts. Ordinary text, with no URL or suspicious digits, almost always gets through.

Based on the filtering patterns that SMS aggregators publish for their customers, here is what most often triggers a silent deletion:

• a URL, even shortened, even without https:// — a plain mysite.com/photos is enough;
• an isolated sequence of 4 to 8 digits, read as a hijacked one-time code;
• a phone number or IBAN in the body of the message;
• sensitive keywords linked to a bank, a delivery, taxes, a prize;
• more than 3 to 5 identical messages sent within a few minutes to the same country;
• a very long message split into segments where a single segment goes missing: the phone then displays nothing at all.

That last point ties in with a trap we cover elsewhere: one accent or emoji drops the limit from 160 to 70 characters and multiplies the segments. We explained it in our guide to the accents and emojis that cut an international text short. The more segments there are, the greater the chance a filter intercepts one of them.

How can I tell it's a filter and not an outage?

By changing one parameter at a time. If a neutral message goes through and the same message with a link doesn't, the diagnosis is made.

Here is the five-step test, to be done in order:

1. Send "Test" on its own to the same international number. If it arrives, your route works: the number, the plan and the interconnection are fine.
2. Send a 100-character sentence with no digits or links. If it arrives, the problem lies in the content of the original message.
3. Reintroduce the suspect element on its own: the link first, in a separate message. If it doesn't go through, you've identified the trigger.
4. Wait 15 minutes between each attempt. Firing off attempts back to back activates throughput checking and makes you look like a bot yourself.
5. Test the reverse direction: ask your contact to write to you. If their messages arrive and yours don't, the filter really is on their carrier's side.

If even "Test" doesn't go through, the problem is elsewhere: number format, international option disabled, a prepaid line out of credit. We covered those cases in the article on texts received but impossible to send from abroad and in the one on the international format of a foreign number.

Writing down your attempts genuinely helps, especially if you then have to describe them to customer service. A small spiral notebook sitting next to the phone, with the exact time and content of each attempt, beats reconstructing it from memory three days later.

How do I get a message containing a link through?

By not sending it as an international text. URL filtering is the most systematic and least negotiable rule: no formatting trick gets around it for long.

The workarounds often recommended vary widely in quality:

The real answer is decoupling: the text carries the alert ("check your email, I've sent you the link"), another channel carries the link. It's also the recommended practice on the business side, where sending links in cross-border texts now requires prior declaration with many carriers.

If your contact is reachable over RCS, the problem disappears: the message travels over the internet rather than the SMS interconnection. RCS does have to be active on both ends, though — we compared the two technologies in RCS versus SMS: why texting is declining in France.