How does SparkPage handle SMS opt outs (STOP)
People opt out by replying "STOP" to any message. It's a standrad format for any SMS communications (and a legal requirement). So our customers usually include "Reply STOP to optout" at the end of their messages.
Currently SMS can be delivered through two 3rd party integrations, Twilio and Nexmo. So you don't need to do anything special with your SparkPage account.