On paper, activating the WhatsApp Business API is "register a number, get a token, send messages." In practice the things that break are approval, number identity, and message templates, and none of them show up in a quickstart. I recently wrote a full activation walkthrough for local businesses in the Dominican Republic (in Spanish), and this is the engineering-level version of what actually trips people up.
Coexistence mode quietly rewrote the migration story
Until recently, connecting a number to the API meant giving up the WhatsApp Business app on the phone and, worse, the conversation history tied to it. Meta's Coexistence feature, rolled out toward the end of 2025, lets a single number run on the mobile app and the Cloud API at the same time. For a developer that is a real architectural change: your automation reads and writes through the API while the business owner still replies from their handset. The catch is collision. If your bot and a human both answer the same thread, the customer gets two replies. Build an explicit handoff state so the bot backs off the moment a human takes over.
Embedded Signup is the only onboarding path worth using
Do not hand-roll onboarding by collecting phone numbers and business details yourself. Meta's Embedded Signup flow handles eligibility, the Business Manager link, and number verification in one QR-driven step. It expects a recent WhatsApp Business app, 2.24.17 or newer, with camera access, a Facebook page linked in the app settings, and a Business Manager account that actually belongs to the business. Skipping any of those is the most common reason a signup stalls halfway.
Template messages are where the bill and the rejections live
Inbound messages are free and unlimited. The moment you initiate a conversation, you are in template territory: pre-approved message templates, sorted by category such as marketing, utility, and authentication, each billed per conversation rather than per message. Two things follow from this. First, budget by conversation category, because marketing costs more than utility. Second, treat template approval as a pipeline with a rejection path, since Meta rejects templates for vague reasons and your send code needs a fallback when a template is not approved yet.
Number reputation is a runtime concern, not a setup checkbox
The fastest way to get a freshly activated number blocked is to blast a few thousand cold messages on day one. Meta assigns every number a quality rating and a messaging limit that scales up as the number proves itself. Warm the number up, watch the quality rating through the webhook, and back off automated sends when it drops. Reputation is something your system has to defend continuously, not a box you tick during setup.
Test the sync window before you promise a go-live date
Activation is not instant. The number typically finishes syncing within 24 to 48 hours, and until it does, sends behave inconsistently. Bake that window into your rollout: verify with a test send, confirm inbound webhooks are arriving, and only then flip the client to live. Promising same-day activation is how you end up debugging Meta's queue instead of your own code.
The short version
Getting a token is the easy 10 percent. The real work is coexistence handoff, Embedded Signup prerequisites, a template pipeline that expects rejections, and defending number reputation at runtime. If you want the business-facing companion to this, the activation requirements and coexistence details written for owners rather than engineers, I put the full guide here: Activar WhatsApp API en Republica Dominicana.
This article was drafted with AI assistance (Claude) and reviewed and edited by me.
