I think different jurisdictions have different police policies in that regard. One thing in particular that we want to avoid is people making the call and running away. We want them to stay on the scene so that they can help the person who's undergoing the difficulties, direct first responders to where the victim is, and proactively inform first responders of what might be the substance involved.
This is an encouragement to remain on the scene. It's not a compulsion. However, if they do leave, they don't meet the criteria. If they leave, they don't have an immunity. If they remain, they get the immunity.