Hey, Richard!
Take a look at this video from the Developer Engagement folks. Around half-way through, he sets up something like this for SMS when someone doesn't answer... (Not EXACTLY your situation, as this is an agent dialed call and you are looking at campaigns, but it's a start!)
As an aside, I highly recommend this channel for anyone "in the weeds" with Genesys Cloud.
HTH
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Paul Simpson
Eventus Solutions Group
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