As per the Carrier regulations, US Long Codes should not be used for A2P SMS. It should only be used for P2P SMS.
The following are the usual characteristics of A2P SMS, which when present will be blocked by carriers:
When more than 200 SMS/day sent per Long Number
When the ratio of incoming to outgoing SMS on a Long Number is more than 1:3
When more than 1 SMS/second/Long Number sent out
When the same message sent to the same user within 5-10 seconds of the first message.