CheckNumber.AI 
Sideline Checker
sideline $1.5 per 10,000 checks
- Core data available
- Number, activated
- Best for
- Flagging secondary or business-use numbers before outreach
No-code CSV REST API Exportable results
Check Phone Numbers for Sideline Registration Signals
Run a focused Sideline registration check across phone-number lists and keep every result mapped to its source record.
Sideline provides a second phone number layered on top of an existing device, commonly used for business lines, classifieds, or keeping work and personal calls separate without a second SIM. CheckNumber's Sideline checker confirms whether a number holds a registered Sideline account, useful for spotting secondary or business-use numbers in a contact list before outreach, without ever contacting the number.
E.164
Phone input
$1.5
Per 10,000 checks
CSV + API
Workflow
Submitted number +1646••••0298
Sideline status yes / no
Sideline Checker 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.
What CheckNumber detects for Sideline
Sideline account by phone
Confirms whether a phone number has a registered Sideline account (yes or no).
Secondary-line flag
Every number in a Sideline batch returns a registered or not-registered result for flagging secondary-line numbers.
Passive, no contact
Reads the Sideline registration signal only — the account is never messaged, logged in to, or notified.
What You Can Check
A focused Sideline registration check
Use the Sideline task when the required output is a row-level registration signal mapped to each submitted E.164 phone number.
| Sideline product | Task type | Input | Core data available | Best for | Current price |
|---|---|---|---|---|---|
| Sideline Checker API documentation | sideline | Phone numbers in E.164 format | Number, activated | Flagging secondary or business-use numbers before outreach | $1.5 per 10,000 checks |
Core Sideline data
What the Sideline task returns
The result file maps one row to each submitted number with the fields below, following the same task/poll/export flow used across CheckNumber's bulk checkers.
Registration
sideline - Number
- The phone number supplied for the check, in E.164 format.
- activated
- Whether the number is registered on Sideline (yes or no).
The activated field is a boolean signal only — it does not include the primary number it's layered over, or other account details.
Run a Sideline batch in three steps
Use the no-code platform for uploaded lists or the REST API for recurring workflows.
- 1
Prepare E.164 numbers
Provide one international phone number per row with country code.
- 2
Create the sideline task
Upload the file or call the API with task_type set to sideline.
- 3
Export mapped rows
Download the completed file and match each result to its source number.
Common Use Case
Improve Sideline list quality before outreach
Clean customer records, segment authorised contact lists, or add a Sideline registration signal to an internal data-quality workflow before analysis.
No-code CSV REST API Exportable results
Signal 1
Secondary-line signal — confirm in bulk which numbers hold a registered Sideline account.
Signal 2
Cleaner outreach lists — flag secondary or business-use numbers before a primary-line-focused campaign.
Signal 3
Privacy-safe — a passive check that never messages, logs in, or notifies the account holder.
Sideline FAQ
Can I check phone numbers against Sideline in bulk?
Yes. Upload a CSV of up to hundreds of thousands of numbers or call the REST API, then download one registered or not-registered result per number. You are billed only for successful checks.
Why would a list need a Sideline check?
Sideline numbers are typically secondary lines layered over a primary phone, often used for business or classifieds. Flagging them helps teams distinguish a person's primary line from a secondary one before outreach.
Does the check contact 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.
Is there a free trial?
Yes. New accounts receive 1,000 free checks on signup with no credit card, and every check afterward is pay-as-you-go with no monthly fee — top-up credit never expires.