A validation API demo always looks great. The trouble shows up later, when “valid” numbers turn out to be ported, landline, or dead. The thing separating a useful validation API from a glorified format checker is rarely on the marketing page. Some return validity, carrier and line type but nothing on porting or whether the line is still in service. The useful ones cover those too and tell you what to do with the result. Here are the seven capabilities I check before committing to one.
1. Real-time carrier data
The API should verify a number against live carrier network data, not a database snapshot from last quarter. That is what gives you accurate routing, current line type, and better delivery rates, and it matters most in places with active number portability. CheckThatPhone runs a live carrier dip at lookup time; see carrier lookup for what comes back.
2. Line type detection
You want to know which of these you are holding:
- Mobile, which can receive SMS
- Landline, which usually can’t (about 7% of the landlines we check are SMS-capable)
- VoIP, which is often where masked or throwaway numbers hide
That split decides which channel to use and how much to trust the number. CheckThatPhone returns line type and carrier sub-type in every API response.
3. A deliverability signal
Good services hand back a usable signal, not just raw fields. CheckThatPhone returns deliverable ("true" or "false") and an action recommendation: "send", "unsubscribe", or "error" for a malformed request. That is the difference between burning a send on a dead number and skipping it.
4. GeoIP and timezone support
Location data lets you send at a reasonable local hour, match an IP to an area code when you are hunting fraud, and aim your targeting more precisely.
5. Portability and deactivation checks
The better APIs track whether a number has been ported, name the carrier it sits on now, and flag recently deactivated lines. If you run fraud screening or 2FA, this is the data that keeps codes from going to dead numbers. A deactivation tells you a number went dead; it can’t tell you who holds the number now, so keep your own records if reassignment matters to you.
6. Optional compliance and reachability add-ons
The base validation tells you whether a number is real and what kind of line it is. A few optional checks decide what you are allowed to do with it and whether you are throwing away contactable numbers. All are opt-in and cover US and Canada numbers; the two paid ones cost one extra credit per lookup, and the DNC checks are free.
- TCPA litigator scrub. This checks each number against a database of known serial TCPA plaintiffs and repeat filers, refreshed daily. On a match it returns the litigator’s name and type and rewrites the response
actionfield to “unsubscribe,” so a single field tells your dialer or send logic to skip the number. If you run outbound calling or texting, this is cheap insurance against the people who make a living off accidental contact. - State DNC & complainer checks. Two free flags (
dncState,dncComplainer) that run a state do-not-call check (38 states plus DC) and a national list of telemarketing complainers. Each check reports whether it ran, so an unchecked state never reads as clear. The federal registry is not included — scrub it separately. - Landline SMS reachability. About 7% of the landlines we check can receive texts, so discarding every landline means dropping contactable people. This check identifies which landlines can actually receive SMS. Mobile numbers skip the check, so you only pay where it applies.
Because all of these are opt-in, you turn them on for the numbers and channels that need them and leave them off everywhere else. See pricing for per-lookup cost.
7. Scalability and speed
Real-time validation needs fast responses, a batch path for lists, transparent pricing, and an SDK you don’t have to fight. Before you commit, confirm the latency and throughput hold up at your actual volume. CheckThatPhone has a real-time endpoint, bulk CSV upload for files up to 1M rows, official Node.js and Python SDKs, and usage-based pricing from $0.005 down to $0.0009 per lookup.
Where to start
When you are evaluating options, weigh accuracy, data freshness, line type clarity, deliverability signals, geographic context, the compliance and reachability add-ons you actually need, and integration speed. The right API earns your trust in every number that passes through it. For what each provider actually charges, see the phone validation API pricing comparison, and for a feature-by-feature view against Twilio Lookup, Telnyx, and Numverify, see CheckThatPhone vs other phone validation APIs.
Browse the CheckThatPhone documentation to review the full set of features and response fields, or see pricing plans to find the tier that fits your validation volume.