What it means
Before a phone number can be enabled on the WhatsApp Business API, Meta verifies that the business controls it. The process: you submit the number through your BSP, Meta sends a verification code via SMS or voice call to that number, you enter the code, and Meta marks the number as verified.
The number must not be currently active on the regular WhatsApp Business app or consumer WhatsApp. If it is, you must first delete the existing WhatsApp account on that number before verification can succeed.
For business verification (a separate step that unlocks higher tiers and additional features), Meta also requires documents proving the business's legal existence: incorporation papers, utility bills, etc.
Why it matters
Phone number verification is the most common stuck-point in WhatsApp onboarding. The number is in use elsewhere, the SMS does not arrive (especially for international SIMs), the business documents are missing or in the wrong format. Each one delays go-live.
The single biggest piece of advice: pick a phone number that is dedicated to the WhatsApp Business API from day one. Sharing a number with personal use, or migrating an existing number that has years of consumer history, creates avoidable problems.
Example
A new client tries to verify their main office line as their WhatsApp Business API number. The verification SMS goes to the desk phone, which cannot receive SMS. They switch to voice verification: same problem (the line is shared, the call gets answered by reception). Solution: a new dedicated number, verified in 5 minutes via voice on a mobile.