Long SMS messages are actually a series of standard length SMS messages containing parts of the message and concatenation instructions. The recipient phone uses these instructions to reassemble the message and present it as one long message.
EMS manages the splitting and submission of the messages to the mobile network to ensure robust delivery.
In order to send long messages via the SMS API, you must first enable the option on your account in the API/SDK section of the Options page of Web SMS. You can then submit messages with text longer than 160 characters.
The maximum number of message parts is 4 giving a maximum message length of 612 characters. There are certain characters in the GSM Alphabet that require two 7 bit values to describe, eg: €. These will count as two characters when the messages are constructed. Any messages longer than 4 parts are truncated at 4 parts.