SMS subscription does not currently support double opt-in.
In order to subscribe a user to receive Text messages (SMS) you will need to push an update event with the following:
{"update":{"isSubText": true}}Also, is worth mentioning that “tel” attribute must have the user phone number. The format for the number should be in international format.
A phone number in full international format includes a plus sign (+) followed by the country code, city code, and local phone number.
Example:
+1 408 XXX XXXX
+4075 XXX XXXX
+41764.XXX.XXX