When evaluating a phone validation method, you’ll encounter different approaches to determining whether a phone number is active and reachable. Traditional methods like HLR (Home Location Register) lookups and LRN (Location Routing Number) lookups have been industry standards for years. However, they weren’t designed to answer the #1 question most businesses actually need answered: Will someone pick up when I call this number?
This article explains how these technologies work, what they can and can’t tell you, and why Trestle’s Phone Activity Score provides a much more comprehensive signal for lead contactability and customer verification.
What is an HLR Lookup? #
HLR lookups query a mobile network’s central subscriber database (called the Home Location Register) to retrieve information about a phone number. In modern LTE/5G networks, similar subscriber status information is maintained in HSS systems, though commercial ‘HLR lookups’ still commonly refer to signaling-based subscriber queries.
When you perform an HLR lookup, the request travels either to the mobile network operator (MNO) or to a signaling network (typically via SS7 or signaling intermediaries), to retrieve subscriber status information.
HLR lookups can tell you:
- Whether a number is currently assigned to a subscriber
- The current network operator serving that number
- Whether the number has been ported to a different carrier
- Basic status (active, absent, or dead)
What HLR lookups cannot tell you:
- Whether someone actually uses the phone regularly
- The likelihood that someone will answer when you call
- Activity patterns over time
- Whether the number belongs to a secondary or rarely-used device
HLR lookups were originally designed for network routing and mobility management, not for predicting whether a lead will convert or a customer will be reachable. They provide a binary, point-in-time status that doesn’t account for user behavior. In other words, HLR lookups can indicate if a number is currently registered and reachable on the network (in other words, “absent” versus “active”), but does not provide connectivity signals, usage evidence, or activity history.
A simple way to remember what HLR lookups can and can’t do:
→ An HLR lookup answers, “Can the network route to this number right now, and on what network/roaming state?”
→ An HLR lookup does NOT answer, “How does this person behave with their phone?”
What is an LRN Lookup? #
LRN (Location Routing Number) lookups serve a different purpose entirely. When the U.S. Congress mandated local number portability in 1996, carriers needed a way to route calls to numbers that had been transferred between providers. The LRN system was created to solve this routing problem.
LRN ensures that a unique 10-digit number is assigned to each telephone switch. Therefore, when a call is made to a ported number, the originating carrier queries a Local Number Portability (LNP) database to get the LRN, which tells the network which switch currently serves that number.
LRN lookups can tell you:
- Which carrier currently serves a phone number
- Whether a number has been ported
- The network switch serving the number
What LRN lookups cannot tell you:
- Whether the number is connected or disconnected
- Whether the phone is actively used
- Any information about call activity or user engagement
LRN data is fundamentally about routing infrastructure, not about whether someone will answer when you dial.
How is Trestle’s Activity Score Different? #
Trestle’s Activity Score approaches phone validation from the perspective that matters most to businesses: Is this number worth calling?
Rather than providing a simple binary status, our Phone Activity Score delivers a 0-100 rating based on actual phone activity observed over the past 12 months. A score of 100 indicates consistent activity, while a score of 0 indicates a number that is likely disconnected, or no observed activity.
Phone Activity Score incorporates multiple data sources that standard HLR and LRN lookups don’t access:
- Call connectivity data from carriers and cross-carrier network enablers
- Phone metadata including line type and carrier assignment
- Name-to-number linkage data and duration of that linkage
- Connection and porting status signals from carriers
- Verified call activity observed across Trestle’s network
These network-level signals provide strong coverage in North American and growing international coverage. HLR lookups are one input that inform our Activity Score, but by combining HLR data with the variety of behavioral and network-level signals listed above, our solution is far more robust than any single lookup method.
What makes this approach fundamentally different is that a Phone Activity Score evaluates behavioral patterns, not just technical status. A phone number might pass an HLR lookup (it’s assigned to a subscriber) but still have an Activity Score of 30 because no one has used it in months.
When Does This Difference Matter? #
The distinction between these approaches is critical to understand in specific business scenarios. Here’s a few examples:
Lead Prioritization #
You’re calling leads, and you want to reach people who are more likely to answer their phone. A number that’s technically valid but rarely used wastes agent time. Trestle recommends prioritizing numbers with Activity Scores of 70 or above for outbound campaigns, as these indicate high confidence that someone will answer.
Fraud Prevention #
Fraudulent sign-ups tend to use numbers that are technically valid but inactive or associated with temporary services, like non-fixed VoIP. This is a scenario where HLR lookups have a significant blind spot. HLR does not support VoIP numbers and typically returns an unknown or “Not Applicable” response. A low Phone Activity Score, by contrast, can reveal a lack of genuine usage patterns across all line types, helping flag suspicious sign-ups regardless of whether the number is mobile, landline, fixed VoIP or non-fixed VoIP.
SMS Campaign Optimization #
For SMS delivery, you need more than carrier routing information. Numbers with low activity scores may technically receive your message, but are very unlikely to engage with it.
What Can’t Activity Score Do? #
No phone validation method provides perfect information, and it’s important to understand the limitations:
- Some regions, like Canada, have less data available for Activity Score calculations. This may affect score accuracy for certain numbers.
- Users who primarily communicate through FaceTime, WhatsApp, or Messenger generate less traditional phone activity, which could result in lower scores even for numbers they actively use.
- Secondary phones or numbers used only occasionally may have lower scores despite being actively monitored by their owners.
Trestle’s Phone Activity Score has an accuracy rate with false positives below 1% for identifying disconnected numbers, but edge cases exist, which is why the score provides a range rather than a binary verdict.
Advantages and Limitations of Each Method #
One of the most overlooked limitations of HLR lookups is that they are inherently restricted to mobile numbers. HLR databases are maintained by mobile network operations for SIM-based subscribers; they contain no records for landlines, fixed VoIP, non-fixed VoIP, or toll-free numbers.
In practice, this is a far bigger gap than many businesses realize. Across 3+ billion queries over the past 6 months, roughly 35% of phone numbers Trestle processed were non-mobile. For any business relying on HLR lookups as their primary validation method, that means a third or more of their phone records receive no validation at all.
Trestle’s Phone Validation API validates all phone line types with a single API call. This ensures comprehensive coverage regardless of what kind of phone number a lead or customer provides.
| Method | Best For | Limitations |
| HLR Lookups | SMS delivery verification, mobile number confirmation | Point-in-time only, mobile numbers only, no activity insights |
| LRN Lookup | Call routing optimization, carrier identification | No connectivity or activity information |
| Phone Activity Score | Lead prioritization, contactability prediction, fraud signals | May underweight users of messaging apps |
Trestle’s Phone Validation API combines carrier lookup, line type detection, and the Activity Score in a single API call, providing comprehensive phone intelligence without requiring multiple vendor integrations.
The Growing Risk of Relying on HLR Lookups #
Beyond just technical limitations, HLR lookups face an increasingly uncertain future due to regulatory and security pressures.
SS7 Security Vulnerabilities Driving Tighter Access Controls #
HLR lookups depend on the SS7 (Signaling System 7) signaling protocol, which dates back to the 1970s and was never designed with modern security in mind. Over the past decade, SS7 has been repeatedly exploited for location tracking, call interception, and SMS hijacking, prompting the FCC to begin pressuring carriers to implement tighter security protocols. As carriers lock down SS7 access to reduce their security exposure, the availability and reliability of HLR lookups will likely shrink.
Operators Restricting Access to Signalling Networks #
Mobile operators routinely limit HLR access for certain countries and use cases, meaning coverage gaps are built into the system. In response to the SS7 security concerns mentioned above, these carriers have begun actively restricting who can query their networks.
The future of HLR lookups as a source of truth for phone validation looks dim. The telecom infrastructure they rely on is becoming harder to access. Businesses building their phone validation strategy around HLR will face mounting uncertainty about their long-term reliability and availability.
Trestle’s approach, which uses multiple data sources and proprietary signals rather than relying solely on SS7 queries, provides a far more sustainable foundation for phone intelligence.
How to Interpret Phone Activity Scores for Your Use Case #
Here’s how to apply Phone Activity Score thresholds:
- 70-100: High confidence the phone is connected and regularly used. Prioritize these for outbound contact.
- 50: Not enough signals to determine if the phone is connected or disconnected
- 0-30: High confidence the phone is disconnected or inactive. Filter these from campaigns or flag for review.
For questions about Trestle’s Activity Score or to test phone validation with your data, contact support@trestleiq.com or start a free trial today.