In most cases, this will be caused by your content exceeding 160 characters, the character limit for one SMS. It is possible to send multiple SMS together, formed as a single, longer SMS; this is called a Concatenated SMS. Therefore, depending on how many characters over the single message character limit (160) you are, you will be charged for the additional messages. For example:
a one-part message will allow you to send up to 160 characters - you are charged for 1 message.
a two-part message will allow you to send up to 306 characters - you are charged for 2 messages.
a three-part message will allow you to send up to 459 characters - you are charged for 3 messages.
and so on.
Take a look at the Sinch SMS Product page to learn about Sinch's SMS products, or click here to sign up to our self-service portal and get started and send your first message today!