A plain SMS holds 160 characters using the GSM-7 alphabet. Go over that and it is split into parts of 153, because each part carries a header saying how to reassemble it.
The moment a message contains one character outside that alphabet — an emoji, a curly quote pasted from a word processor, or any non-Latin script — the whole message switches to UCS-2 and the limit drops to 70 characters, then 67 per part. A 150-character message with one emoji in it is three SMS, not one.
The practical rule: type your campaigns in a plain text box, not in a word processor, and watch the segment counter before you send rather than the character count after.