Marichi Talks
The units a long SMS is split into for delivery and billing, set by how many characters fit in one message.
A single SMS has a fixed size. Using the standard character set, one message holds 160 characters. Longer texts are split into parts (segments) and joined back together on the phone, but each part loses a few characters to the joining information, so it carries 153. If the message contains any character outside the standard set, such as many emojis or some accented and non-Latin letters, it switches to Unicode, and the limits fall to 70 for one part and 67 per part after that.
This matters because SMS is usually priced per part. A 161-character message costs the same as two 160-character ones, and a single curly quote or symbol can double the cost of a message.
Write SMS short, check the character count before sending, and watch for characters pasted in from word processors. In Marichi Talks, SMS is priced per message part, so the count you see before sending is the count you pay for.
A 150-character reminder is 1 part. Add a 20-character sign-off and it becomes 2 parts. Add an emoji and the same text may become 3.