I immediately figured this was a phishing attempt to get me to hit send and they'd have my email address or try to get other details in a reply. But then I realized the "to:" email was "firstname.lastname@example.org". Now I'm not saying the phisher can't own that email, but seeing as that email is used all time to where it's basically a default throwaway/test/example address it seems unlikely to me it's still available for someone to grab and use for this.
So what I'm wondering is why throw me into my mail client in this phishing attempt. Is it likely they do own that email or have a way to intercept mail to it? Or were they just using the mail client as a sort of popup with the goal of me to call the number in the subject line?