The number of credits per SMS depends on the message length – an SMS of 160 characters or less is 1 credit, a longer message will take 2 or more credits.
Special characters can trigger an encoding which increases the character count. For example, é, â, ä, ' " and more. Any links to your website or shorthand codes to cancel, reschedule, or join Zoom will also add additional characters to your total count.
To avoid any surprises, check these two links:
This link will tell you if you have any unicode characters in your text message. This link will tell you how many segments your text message will be.