Phone verification at signup is a balancing act, and most teams fall off one side or the other. Make it too heavy and legitimate users abandon the form. Skip it and your database fills with typos, fake numbers, and unreachable contacts that quietly cost you money for months. The version that actually works runs in the background: real-time validation that catches bad data without the user ever feeling it.
Why the signup field matters more than it looks
A phone number captured at registration doesn’t stay put. It flows into your marketing lists, your 2FA system, your sales queue, and your support tools. A bad number entered once compounds across every one of them:
- Marketing: SMS to invalid numbers wastes budget and drags down your sender reputation
- 2FA: OTP codes to disconnected numbers produce frustrated users and support tickets
- Sales: reps lose time on numbers that ring out or reach the wrong person
- Analytics: bad contacts distort your funnel and inflate acquisition cost
Catching the problem at the point of collection is dramatically cheaper than cleaning it out of four systems later.
Validating without adding friction
The goal is to check the number without making the user work for it. A few patterns that hold up in production:
Validate on blur or submit
Call the API when the user tabs out of the phone field or hits submit, not on every keystroke. You get a complete number to check and you avoid laggy, half-typed validation while they’re still entering digits.
Show inline feedback
If the number is invalid, undeliverable, or a landline when you need mobile, show a plain inline error: “Please enter a valid mobile number.” It reads like normal form validation, which is exactly what users expect.
Accept VoIP selectively
Not every VoIP number is fraud. Google Voice and business VoIP lines are legitimate, and blanket-blocking VoIP will reject real users. Use the carrier data to make a finer call: flag known disposable carriers, let established VoIP providers through.
Don’t make validation the only gate
Phone validation should inform your signup logic, not be the entire decision. Pair it with email verification, CAPTCHA, and behavioral signals so you catch fraud in layers instead of leaning on one check that’s easy to slip past.
What the API hands back at signup
Call CheckThatPhone’s API with a number and you get:
- Line type: mobile, landline, or VoIP
- Deliverability: whether the number can take calls and SMS
- Carrier: the operating carrier and network type
- Deactivation status: whether the number is currently active
- Action recommendation: “send”, “wait”, or “skip” based on status
See the full response format to map these fields into your signup logic.
Measuring whether it worked
Capture these before you turn validation on, then watch them after:
- Invalid number rate: share of signups with bad phone data
- 2FA delivery rate: share of OTP codes that actually land
- Sales contact rate: share of leads who answer or reply
- Support tickets: volume of “I never got my code” complaints
If validation is doing its job, all four move in the right direction, and the invalid-number rate is usually the first to drop.
Next step
You don’t need OTP codes and extra screens to get clean phone data. Real-time API validation catches bad numbers silently and keeps the signup flow fast. Read the API documentation to add it to your registration forms, or explore pricing plans and start on the free tier.