Messaging limits determine the maximum number of business-initiated conversations (with unique contacts) each phone number can start in a rolling 24-hour period.
Meta’s WhatsApp Business API has some native rules and laws to make sure the quality of conversations over their platform stays high.
All phone numbers with an approved display name and a verified business start with 1K business-initiated conversations with unique contacts in a rolling 24-hour period. If the messaging limit is reached, you can start more conversations as soon as one or more active conversations end.
Each time a business-initiated conversation is initiated, Meta will determine if the Tier Limit should be increased. This determination is based on the following criteria:
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The WhatsApp Business API Phone number status is ‘Connected’.
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The WhatsApp Business API Phone quality rating is ‘medium’ or ‘high’.
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In the last 7 days, an X amount of conversations is initiated with unique contacts, where X is the current messaging limit divided by 2.
If all these conditions are met, whereby the third is most relevant, the Tier Limit will be increased by one level in 24 Hours. There are four levels:
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Tier 1: 1K unique customer
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Tier 2: 10K unique customers
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Tier 3: 100K unique customers
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Tier 4: An unlimited number of unique customers
Decreasing in Tier Limit
Each time a new conversation is initiated initiated with a unique customer Meta will check your phone number quality rating. If the rating has been Low for the last 7 days, Meta will immediately decrease your messaging limit by one level.
Source: https://developers.facebook.com/docs/whatsapp/messaging-limits