In the UserHeader specification for an SMS PDU, there is a portion in the UserHeader called the modulus which is a number that can run from 0 to 255.
This modulus value is managed by the sending party.
1st Concat Message:
2nd Concat Message:
The modulus value combined with the mobile number means every mobile number can receive up to 255 different incomplete long concat messages without risk of messages being jumbled up. If the same modulus value is used repeatedly, then there is no control over how the handset will re-assemble the message.
Do note that the reassembly of long message parts is handled by each handset model's software. The cloud service is not able to determine if the reassembly of long message parts by the handset will be correctly done.
In use cases where long concatenated messages are sent to the same mobile recipient repeatedly, there is a small change that wrong messages parts are joined together into a long message by the recipient device. In such uses cases, we recommend using WhatsApp and Push channels to deliver the long message content.