Mo,
You can do this directly today in Genesys Cloud without an agent script using the channel switching feature:
https://help.mypurecloud.com/articles/switch-channels-for-an-interaction/. While the agent is handling the chat they can click the '+' icon to switch to the voice channel, assuming the customer has a number registered on their contact record.
If you aren't using Genesys External Contact records then you'd have to create a Data Action to place that call, and then invoke that Data Action from the agent script via a button or link.
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Jim Crespino
Senior Director, Developer Evangelism
Genesys
https://developer.genesys.com------------------------------