Verifying phone numbers at signup is one of the most effective ways to improve your user data quality — but it has to be done carefully. Add too much friction and you lose legitimate users. Skip verification entirely and your database fills with fake numbers, typos, and unreachable contacts that cost you money downstream. The key is real-time validation that works silently in the background, catching bad data without slowing down the user.
Why Signup Validation Matters
Phone numbers collected at registration flow into your marketing lists, 2FA systems, sales queues, and customer support tools. Bad data at this stage compounds across every system:
- Marketing: SMS campaigns sent to invalid numbers waste budget and hurt sender reputation
- 2FA: OTP codes sent to disconnected numbers create frustrated users and support tickets
- Sales: Reps waste time calling numbers that ring out or belong to the wrong person
- Analytics: Invalid contacts distort your funnel metrics and inflate acquisition costs
Catching these issues at the point of collection — before the data enters your systems — is dramatically cheaper than cleaning it later.
Validation Without Friction
The goal is to validate the phone number without making the user jump through hoops. Here’s how:
1. Validate on Blur or Submit
Call the validation API when the user tabs out of the phone field or clicks submit — not on every keystroke. This gives you a complete number to check without introducing lag during typing.
2. Show Inline Feedback
If the number is invalid, undeliverable, or a landline (when you need mobile), show a clear inline error: “Please enter a valid mobile number.” This feels natural and matches standard form validation patterns.
3. Accept VoIP Selectively
Not all VoIP numbers are fraudulent. Google Voice and business VoIP numbers are legitimate. Rather than blanket-blocking VoIP, use the carrier data to make nuanced decisions — flag known disposable carriers while allowing established VoIP providers.
4. Don’t Gate on Validation Alone
Phone validation should inform your signup logic, not be the only gate. Combine it with email verification, CAPTCHA, and behavioral signals for a layered approach that catches fraud without blocking real users.
What the API Returns at Signup
When you call CheckThatPhone’s API with a phone number, you get back:
- Line type — Mobile, landline, or VoIP
- Deliverability — Whether the number can receive 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 the number’s status
See the full response format to understand how to map these fields into your signup validation logic.
Measuring the Impact
Track these metrics before and after implementing phone validation at signup:
- Invalid number rate — Percentage of signups with bad phone data
- 2FA delivery rate — Percentage of OTP codes successfully delivered
- Sales contact rate — Percentage of leads that answer or respond to outreach
- Support tickets — Volume of “I didn’t receive my code” complaints
Most businesses see a measurable improvement across all four within the first month.
Getting Started
Phone verification at signup doesn’t have to mean OTP codes and extra steps. With real-time API validation, you can catch bad numbers silently and keep your signup flow fast. Read the API documentation to integrate phone validation into your registration forms, or explore pricing plans to get started with the free tier.