What an HLR lookup is

Every mobile network keeps a register of its subscribers, called the Home Location Register (HLR). The network uses it to route calls and texts: before a call or SMS is delivered, the network asks the HLR which SIM holds the number and whether it can be reached. An HLR lookup asks the same question on your behalf. Nothing is sent to the phone and it doesn't ring, but the answer tells you whether the number is live right now, which network it's on, and whether it has been ported.

For anyone dialling UK mobiles at volume, that makes it the most direct way to find dead numbers before a rep wastes a dial on them. It also has more ways to mislead than most vendors admit, so this piece covers both: what the lookup is good for, and which of its answers deserve your trust.

How the lookup works

Mobile networks talk to each other over a signalling network (SS7), separate from the calls themselves. A lookup provider connects to it with a signalling address called a Global Title, and sends the same routing query a network sends before delivering a text. The query goes to the network that currently holds the number, which answers from its HLR. Newer 4G and 5G cores call the register something else, but the lookup works the same way.

Two things follow from that. First, the answer is live: it reflects the number's state at the moment of the query, not a database that was compiled weeks ago. Second, the quality of the answer depends on the route. The provider has to know which network holds the number today (porting data), and the network has to accept the query from that provider's Global Title. Both vary by provider and by network, and that is where most of the pitfalls below come from.

What comes back

Exact labels vary by provider, but a UK lookup returns some version of:

  • Live (connected): a SIM holds the number and the network can reach it.
  • Absent (switched off): the number is in service, but the phone is off or out of coverage. This is a live number.
  • Dead (unknown subscriber): the network says no SIM holds the number. It was disconnected, or never issued.
  • Undetermined: the query failed, timed out or was blocked. This tells you nothing about the number.
  • Network and ported flag: which network holds the number today, and whether it has moved from the network that first issued it.

Some providers also return the SIM's identifier (IMSI) and the switch currently serving it (MSC), which can reveal the country a phone is roaming in. A telemarketer has no use for either, and storing them adds personal data you then have to justify. Ask for them to be left out.

How telemarketers use it

  • Cleaning a list before a campaign. Run a bought or aged list through lookups, suppress the dead numbers, and your connect rate and agent time reflect real people. On old data the dead share can be large, and each dead number is a dial that can never convert.
  • Checking a lead at capture. A web form lead with a mistyped or made-up mobile can be caught when it arrives, before it reaches a dialler queue or a paid lead report.
  • Keeping CRM data accurate. UK GDPR expects personal data to be kept accurate. A dated "mobile status" on each contact is a practical way to show you check.
  • Spotting recycled numbers. A number that was dead and is now live again may belong to a different person. Consent or a customer relationship with the previous holder doesn't carry over to the new one, so treat that record as a new contact.

One thing it does not do is replace TPS screening. A live number can be on the TPS or CTPS register, and PECR applies to it all the same. The two checks answer different questions: can this number be reached, and may you call it for marketing. I've covered the second one in how to check a number against TPS.

Which results you can trust

Dead, from a well-routed query, is the strongest answer the lookup gives. The network that owns the number is saying no SIM holds it. When the provider's porting data is right and the network accepted the query, that answer is rarely wrong.

Live is reliable but generous. A live result means a SIM holds the number, not that anyone still uses it. A prepaid SIM left in a drawer, or a number the network has not yet released, can read live long after the person has moved on. The lookup under-reports dead numbers far more often than it over-reports them. That is the safer direction to be wrong in: you miss a few dead numbers, rather than suppressing good ones.

Absent means live. A switched-off phone is a working number. Suppressing absent results throws away real contacts; call them later instead.

Undetermined means nothing. Treat it as "not checked", never as dead.

The network name is right for the big four, loose below them. Most UK MVNOs (giffgaff, Tesco Mobile, Voxi and others) ride on a parent network and show up as EE, O2, Vodafone or Three. That is a branding gap, not an accuracy problem: whether the number is live still comes back correctly.

The pitfalls

  • Porting errors create false deads. If a provider's porting data is out of date, it sends the query to the network that first issued the number. That network no longer has the subscriber and answers "unknown", so a live, ported number comes back dead. This is the error that costs you real leads, and it is the first thing to test.
  • Blocked queries mislabelled as dead. UK networks filter signalling traffic from outside sources, and some providers' routes are refused. A good provider returns that as undetermined. A weak one maps it to dead. Ask each provider how a blocked query is reported, and check it in testing.
  • Masked answers. Some networks can reply with a generic answer that looks like "connected" without really checking the subscriber. Known-dead numbers in your test set will show whether a provider's results on a given network are real.
  • Charging for non-answers. Some providers charge for every query, including undetermined results and landlines. Others charge only when they get an answer. On a large, messy list the difference is real money.
  • Landlines are out of scope. Fixed lines have no HLR. A provider may return range or porting data for a landline, but not whether it is in service.
  • A result has a shelf life. Numbers are disconnected and reissued all the time. A dead result from six months ago may belong to someone new today, so date every result and re-check before relying on an old one.

What an HLR lookup does not tell you

It tells you a SIM holds the number. It does not tell you who holds it, whether they are the person in your CRM, whether they will answer, or whether you are allowed to call them. Identity needs a separate check against your own records or a data provider; permission needs TPS screening and your consent records.

Choosing a provider

Prices are low, typically fractions of a penny per lookup at volume, so price is rarely the deciding factor. Accuracy is, and it varies by network. Before committing, run a test set of a few hundred numbers where you already know the answer:

  • live numbers on each of EE, O2, Vodafone and Three, plus a few MVNOs;
  • live numbers you know have been ported;
  • a few phones deliberately switched off;
  • numbers you know are disconnected;
  • numbers that were never issued (Ofcom reserves 07700 900000 to 07700 900999 for drama use, so they never belong to anyone);
  • landlines and malformed numbers, to see what you are charged for.

Run the same set through each provider within a few minutes, then score each against the known answer, network by network. The number that matters most is how many live numbers each provider calls dead. After accuracy, ask whose Global Title the UK queries leave on (owned or leased), which statuses are charged, where the data is hosted, how long the numbers you send are kept, and whether they will sign a data processing agreement.

How this will work in TPSClear

We're adding mobile validation to TPSClear, alongside TPS and CTPS screening. We're testing providers now against a known-answer set like the one above, so the details below are the shape we expect rather than a finished spec:

  • A workflow action at lead capture. In HubSpot, a workflow step next to the TPS check validates a new lead's mobile once and writes the result to the contact. The same check will be available through Zapier, Make and the API.
  • A scan of a list or segment. For tidying numbers already in your CRM, pick a list and the results are written to each record, ready to filter before a calling session.
  • A status for every phone field you choose: each one gets its own Liveness Status (live, switched off, dead or undetermined) and Liveness Status Checked At, named after the field, such as Mobile Phone Liveness Status, so a contact with two mobiles has an answer for each.
  • Dead means don't dial, not delete. We won't overwrite or remove the phone number. You decide what a dead result triggers, such as dropping out of a sequence or moving to an email flow.
  • Mobiles only. Landlines are skipped and not charged.
  • No IMSI, MSC or roaming data is requested or stored.
  • Opt-in and priced per check, separately from TPS screening.

If you'd find this useful, I'd like to hear how you would want it to work: on every new lead or only on request, which fields you'd actually use, and what a dead number should trigger in your CRM. Get in touch.

One-line conclusion

An HLR lookup asks the mobile network whether a number is live right now; trust a well-routed dead result, read live and switched-off as reachable, treat undetermined as unchecked, test providers on numbers you already know before you rely on any of them, and keep screening against TPS, because a live number and a callable number are different things.