SMS traffic is heavily restricted when sending to the US. Rules & regulations can be found here.
In summary, if you're using long codes (normal mobile numbers) to send and you're doing any of the following, it's likely your messages will be blocked by the networks:
This is outside of our control. If any messages are blocked, they will still show as 'Sent' as we're not notified by the networks when this happens.
The solution to this is use a shortcode. There are 2 options:
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