An SMS database with 15% inactive numbers gives no warning at the point of entry. The damage shows up in your deliverability rate and on your bill at the end of the month. Verifying a mobile phone number before sending removes dead spend before it piles up. This guide covers the available methods and the right moments to apply them.
What invalid numbers cost you
An SMS sent to an inactive or badly formatted number costs as much as one that gets delivered. No reply, no conversion. The cost line stays the same.
The mobile base turns over constantly. According to ARCEP, the quarterly cancellation rate on postpaid plans fell from 5.0% in Q3 2024 to 3.8% in Q3 2025 (mobile services observatory), which still amounts to about 15% of the base over 12 months. On prepaid, the rate reaches 29.5% in a single quarter. Portability lets people keep their number when they switch carrier. A number cancelled without porting goes back to its carrier and can be reassigned to another subscriber without your database being told.
On a list of 50,000 contacts left unverified for 18 months, the numbers that have gone invalid can add up to several thousand wasted sends per campaign: at 15% inactive, 7,500 SMS go nowhere. Once the deliverability rate drops, carriers turn on filters. Later sends then land less easily, even to valid numbers.
How to check whether a mobile number is active in real time
The reference method is to query the carrier’s network register directly. The HLR (Home Location Register) is the database each mobile carrier keeps for its subscribers. It holds the SIM status and the portability information. A network verification request answers immediately: it tells you whether the number exists and whether it is still active.
The check flags unallocated or deactivated numbers, portability from one carrier to another and the line type (mobile or landline). It is non-intrusive: no SMS is sent to the number being checked and the subscriber gets no notification. Legal teams who question whether this kind of check is compatible with GDPR will want to know that.
Captain Verify offers this service via API or file upload, with announced coverage of 96 countries and more than 300 carriers. Each number gets a “valid”, “invalid” or “unknown” status, along with its network. Unlike a reverse phone directory, this method reveals nothing about the holder’s identity. It only answers whether the number can receive an SMS right now.
E.164 normalization: a step that often gets skipped
Before any verification, the number has to be readable by telecom systems. The E.164 format, defined by the International Telecommunication Union (ITU) in recommendation ITU-T E.164, is the standard for any international SMS.
The structure: a + sign, then the country code (1 to 3 digits), then the subscriber number, with no spaces or dashes, for 15 digits at most. French mobile numbers start with 06 or 07. The number 06 12 34 56 78 is written +33612345678, where the +33 country code replaces the leading 0.
The same number often sits in a database in 4 or 5 different forms. Without normalization, tools return false negatives and SMS platforms reject sends. Google’s libphonenumber library or a CRM import settles this upstream.
When to verify the validity of a mobile number
Ideally at entry, then before each campaign, then during a periodic cleanup of the base.
At the point of entry
A phone field wired to a validation API rejects impossible formats and non-existent country codes before they reach the database. Correcting them costs almost nothing at this stage, and it is the only moment when the degradation stops at the source.
Before each campaign
A pass over the full list catches numbers that went inactive since the last check. On a database left several months without a check, the volume to exclude can be large.
Periodic cleaning
A full database cleanup catches whatever drifted between two campaigns. Plan one pass a month for a high volume of new contacts and one every 6 months for a stable base.
What GDPR says about keeping phone numbers
In force since May 2018, GDPR (EU regulation 2016/679) sets two principles that apply directly to a number database: data minimization (Article 5(1)(c)) and storage limitation (Article 5(1)(e)). A mobile number may only be kept as long as the purpose it was collected for requires. After that, you delete it or anonymize it.
For SMS prospecting, the CNIL sets the retention cap at 3 years from collection or the prospect’s last contact. A click on a link in an email counts as contact and resets the starting point. After the 3 years, the organization may contact the prospect once more to confirm their interest. Without a positive and explicit answer, the data should be deleted or archived. A number nobody has verified since then carries 2 risks at once: reaching a former holder who never agreed to receive your messages, and holding personal data past the permitted period.
Breaches of the Article 5 principles fall into the top fine tier of Article 83: up to €20 million or 4% of annual worldwide turnover, whichever is higher.
Captain Verify combines network activity verification and the detection of numbers to purge in a single operation.
Frequently asked questions
Can you verify a mobile phone number for free?
Some tools offer a limited number of free verifications as a trial. Beyond that, real-time network verification is a paid service, billed by volume. The cost stays below that of an undelivered SMS: for campaigns of several thousand sends, the math quickly works in your favor.
How do you identify a mobile number’s carrier?
Network verification returns the number’s current carrier and country of origin, on top of the active or inactive status. The result accounts for portability: if a subscriber switched carrier, the new one appears, for example Orange, SFR, Bouygues Telecom or Free Mobile. That helps when you segment a database by network or look for foreign numbers in a supposedly domestic list.
Does verification also work for landline numbers?
No. Network verification via the HLR register only covers mobile lines. Landline numbers (01 to 05 in France) run on a different type of infrastructure and cannot be queried the same way. Verification tools do identify the line type (mobile or landline) during the check, so you can filter out landline numbers that slipped into an SMS database.
