https://experienceleague.adobe.com/docs/journey-optimizer/using/configuration/frequency-rules.html
The Frequency Rules currently only allow the use of either Email or Push notification channels. Will they include SMS in the future, or are there any current alternatives?
@johnnyliao, Not sure SMS will be included in the future as part of the frequency rules. Here is something you can try to setup an attribute on the profile and capture the SMS sent using update activity then use that field to filter the profiles if they have received 'X' no of SMS in condition activity to branch them out.
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