Hello Santiago,
Welcome to the Community! To answer your question, I'm going to include a great response from @Robert Wakefield-Carl from a previous post of theirs:
There is no free or default dev or sandbox. You have two options:
1. Get a Dev org
This will costs you a minimum per month and gives you something like 5 or 10 PC3 licenses. You still have to pay for any usage of voice or BYOT
2. Use Divisions
With divisions in your Production ORG, you can separate out flows to their own division and only people in that division will be able to see it. The division can also have their own queues and other parts of the ORG for separation and visibility restrictions. This method allows you to access your existing data actions and prompts. When you are done testing, you just move into production and move into your home division.
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Jason Kleitz
Online Community Manager/Moderator
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