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:

  1. When more than 500 SMS/day sent per Long Number.
  2. When the ratio of incoming to outgoing SMS on a Long Number is more than 1:3 (Outgoing SMS -> Incoming SMS).
  3. When more than 1 SMS/second/Long Number sent out (If you require higher throughput, you can purchase more numbers and spread your traffic across your numbers. E.g. 10 numbers = 10 SMS per second).
  4. When the same message sent to the same user within 5-10 seconds of the first message.
  5. When using repetitive URL's and keywords, shortened links(such as bit.ly) in the SMS content


Note: When sending SMS to US destination numbers please use your US long-code number(from Plivo account) in international format. If not, you will get this error: "this caller id is not allowed for outbound message".