Hi James,
Your doubt is perfectly valid here since the English translation doesn't change, but the Spanish structure does. Actually what happens in the second sentence is that it is not expressing a real future in Spanish, but rather an immediate action. Sometimes you can use a present tense in Spanish to express something that you're planning to do right away. So to translate more precisely, we could say:
Voy a llamar a Sandra = I'm going to call Sandra > near future
Yo llamo a Sandra (enseguida) = I'm calling Sandra (right away) > immediate action
To avoid further confusions, we'll make sure the translation in English matches -even if it's not the most natural- in the next update. Thanks for pointing out this issue and enjoy the rest of level 5!