01
Upload your list
Import CSV or TXT files with phone number data, then confirm the task settings before processing.
Sideline Checker
Detect whether phone numbers are registered on Sideline.
Price (per 10,000 numbers) $1.5 USD
Detect whether phone numbers are registered on Sideline.
Detect whether phone numbers are registered on Sideline.
Price (per 10,000 numbers) $1.5 USD
Detect whether phone numbers are registered on Sideline.
No account, number, or SIM required
Every check runs through official platform APIs on our own infrastructure. You never connect, bind, or use your own account, phone number, or SIM card — just upload your list or call the API and get results.
- Bulk CSV and API workflow
- Clear, export-ready results
- Passive check without messaging the account
- Pay-as-you-go processing
Detection signals
What Sideline Checker detects
Sideline Checker confirms whether a phone number is registered on Sideline, the North American second-phone-number app. Verification and fraud teams use it to flag Sideline virtual numbers among contact lists, without messaging the number or logging in to any account.
Registration status
Confirms whether the phone number is registered on Sideline (yes or no), so you can filter out unregistered numbers before outreach.
Virtual-number screening
Flags numbers that are Sideline virtual lines rather than a primary carrier number, useful for spotting burner or second numbers in identity-verification and lead-quality workflows.
Passive, no contact
Reads registration signals only. The account holder is never messaged, logged in to, or notified.
Workflow
How Sideline Checker works
Use Sideline Checker as part of a repeatable list-cleaning workflow before outreach, CRM imports, or audience segmentation.
02
Run the check
NumberChecker.AI processes the file in bulk and returns structured status signals for each row.
03
Export segments
Download results and route clean, matched, unmatched, or review-needed rows into your next workflow.
Outputs
What you can review after the check
number
activated
Use cases
Where teams use Sideline Checker
List cleaning
Remove unsupported or low-confidence records before campaigns or imports.
Audience segmentation
Create groups based on platform, email, phone, or account-related signals.
Review workflows
Send uncertain or high-value records into a separate internal review queue.
Related checks
Pair Sideline Checker with related products
Explore checks that teams often run before or after this product.
Phone Number Validation
Validate numbering format, line type, and reachability signals before platform-specific checks
Number Active Checker
Detect phone number activity level via big-data analytics to improve SMS delivery and reduce costs
High-value Users Checker
Identify high-value phone users via big-data signals for better targeting and prioritization
E-commerce Activity Checker
Filter out numbers recently active on e-commerce platforms and reach high-converting users with real purchasing habits.
FAQ
Sideline Checker questions
Can I check phone numbers against Sideline in bulk? +
Yes. Upload a CSV of numbers or call the REST API, then download a result file with a registered or not-registered verdict for every number. You are billed only for successful checks.
Does the check message or notify the person? +
No. The check is passive and reads registration signals only. Nothing is sent to the number, and the account holder is never contacted or notified.
Where can I find the full list of return fields for Sideline Checker? +
Sideline Checker does not have a dedicated field-reference page. For general API usage, authentication, and response format shared across products, see the documentation at https://docs.checknumber.ai/.
Is there a free trial? +
Yes. New accounts receive free checks on signup, and every check is pay-as-you-go with no monthly fee.
Ready to run Sideline Checker at scale?
Compare pricing, review API options, or create an account to upload your first production list.
