SMS messages has a maximum length, which depends on the encoding.
Default SMS encoding is GSM7; it supports all English letters, numbers and main signs, but does not support emojis or characters like ы or ß. When you use even a single character out of GSM7, then the system has to switch to UCS2.
The maximum message size in GSM7 is 153 symbols, in UCS2 – 67 characters.
If your message is longer than that limit, then the system split it in smaller chunks and then merge back on a mobile phone. If your message consists of 2 fragments, then it's price will double.
Member Signal estimates the message length when you editing campaign content. However if you use merge tags, the final message length will be known only after the message is sent to recipient. Learn more about merge tags.