Text messages were designed around a limited alphabet. Anything outside it needs a different encoding, which fits fewer characters per part. That single fact explains most questions about cost and appearance.

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Characters per part

A message is split into as many parts as it needs, and each part is charged.
Mixing a single non-Latin character into an otherwise Latin message drops the whole message to 140 characters per part. One stray accented character can push a 155-character message into two parts.

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Greek is converted, and the conversion is visible

Greek text is mapped into the alphabet phone networks carry for it. Two things happen in that mapping:
  1. Everything is folded to uppercase. Lowercase and accented Greek both become capitals.
  2. Letters that look identical to Latin ones are mapped onto them. Alpha becomes A, Beta becomes B, and so on, because they are visually indistinguishable on a handset.
A Greek message does not arrive exactly as you typed it. Your recipient sees uppercase text. There is no setting to change this, so write Greek messages expecting capitals rather than treating it as a fault.
Letters with no Latin twin — Gamma, Delta, Theta, Lambda, Xi, Pi, Sigma, Phi, Psi, Omega — are carried as themselves, still uppercase.

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Emoji and other symbols

Emoji count as two characters each. A message with several of them reaches a part boundary sooner than the character counter suggests. Add them, then check the part count.

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The counter on screen is a guide

The character counter counts every character as one and assumes 160 per part regardless of alphabet. It is a quick guide for ordinary Latin text and it can disagree with what you are billed for anything else. The part count is the number to trust. It is shown beside the message box on Send SMS, and the exact figure charged is recorded on each transaction in Billing and usage.

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Line breaks

Line breaks are preserved and count as characters. A message laid out over several short lines can be longer than it looks.

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Practical guidance

Latin text under 160 characters is one part and the cheapest option available. If a campaign is close to the boundary, the highest-value edit is trimming under it rather than anything else you could change.
Budget 140 characters per part, and write knowing the result arrives in capitals. Since it will be uppercase anyway, do not spend effort on capitalisation or accents.
Variables are substituted before the message is measured, so a long name pushes that recipient’s message into an extra part while shorter names stay in one. Different recipients in one campaign can cost different amounts.Size your message against your longest expected value, not a typical one.

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Confirming what a message was charged as

The authoritative record is the part count on the transaction in Billing and usage. It records the number of parts and the cost per part applied to that message.