For SMS message content longer than 160 characters, you must split the message to max 153 characters per post and send as concatenated messages. Most mobile phone would display this as a single long message.
Each SMS consists of 160 characters as defined by the GSM specification. For long messages, each 153 characters will be considered as 1 message. The remaining 7 characters are used as "technical data glue" to join each message part together.
When you send a concatenated msg, you need to be careful for UserHeader value.
Below is an example Userheader value for each part of of a 3-part message.
where the Userheader value is
050003 ==> constant value
76 ==> identify code (counter value)
03 ==> total msg part
01 ==> current msg part
It is recommend that your system to assign different value for identify code (counter value).
The range is from 0 to FF in hexadecimal.
You can refer to section 3.2 of our API guide for the detailed instructions.
Attached here is sample code in PHP for concatenated messages.