The first time I shipped an SMS flow that texted landlines, nothing errored. The messages just vanished. No bounce, no warning, and a billing line item for every send. That’s the trap with line type: the wrong kind of number doesn’t fail loudly, it fails silently and bills you anyway. Knowing whether a number is mobile, VoIP, or landline before you reach out is what separates a clean campaign from a slow budget leak.
The three line types you’ll deal with
When a user hands you a phone number, it almost always falls into one of three buckets.
Mobile (wireless)
Tied to a SIM or eSIM. It takes SMS and voice, it’s usually someone’s primary contact, and it’s the only type you can rely on for 2FA and marketing texts. This is the number you actually want.
VoIP (voice over IP)
Hosted by internet-based carriers like Google Voice, Skype, or Twilio. It may accept SMS, but not always, and not always reliably. VoIP shows up everywhere from legitimate business lines and call centers to disposable numbers used to mask identity or farm signups. The mixed nature is what makes it tricky to handle.
Landline
Fixed-location numbers: home phones, office lines. They cannot receive SMS, full stop. They’re still common across enterprise, healthcare, and government, so you can’t just assume everyone is on mobile.
Why the distinction is worth checking
Aim at the wrong line type and a few things go wrong at once:
- Failed SMS delivery. Landlines and many VoIP numbers don’t support texting, so the message dies quietly.
- Wasted spend. You still get charged for the undelivered message or the failed call attempt.
- Skewed analytics. Bad delivery data distorts how your campaign looks, so you optimize against noise.
- Compliance risk. Some rules, TCPA in the U.S. included, restrict auto-dialing certain line types without consent.
Identify the line type up front and you decide how, or whether, to reach out. That decision is most of the value.
What this looks like in practice
Promo texts: target mobile only. Texting a landline burns budget and never lands.
2FA: mobile is the only type that reliably delivers one-time codes. VoIP introduces delays and spoofing exposure, and landlines won’t receive a code at all. If your OTP delivery rate is sagging, check what share of your numbers aren’t mobile.
Lead scoring and fraud: a VoIP number on a signup isn’t proof of fraud, but a disposable VoIP carrier paired with a burst of form submissions usually is. Flag those and let sales spend time on real leads.
Compliance: in the U.S. you need express written consent to call or text certain numbers. Line type is one of the inputs that keeps you on the right side of TCPA.
How to detect line type automatically
Plenty of teams try to infer line type from area codes or carrier names. It doesn’t hold up. Numbers port, people relocate, and that approach goes stale fast.
CheckThatPhone uses real-time carrier data and direct network queries to return line type for every U.S. and Canadian number, along with:
- Line type (mobile, VoIP, landline)
- Carrier name and subtype
- Routing and portability info
- Deliverability indicators
See the API documentation for the full response format and integration examples. From there you can wire filtering, scoring, or routing logic straight into your CRM or marketing automation.
Next step
Line type is a small field that decides whether a message arrives, what it costs, and whether you’re compliant. Add the check at the point you collect a number, branch your sends on it, and the silent failures stop. Read the documentation to integrate line type checks, or explore pricing plans to validate at the volume you need.