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Our API throttling limits are not associated at the account level, but are based upon each access_token which is considered an authenticated user (an extension in RingCentral which is used to authenticate and obtain an access_token using a given set of API Keys).
40 * 60 (minutes in hour) * 24 (hours in day) * 30 (rough number of days per month) = 1,728,000
If you need to send 2M SMS messages/month, you would need to have two authenticated users (two user extensions in RingCentral each used to authenticate and obtain an API access_token) on the Basic Usage Plan.