Question: how to flirt with a guy talking on the phone?
Answer: It's the man of your dreams calling and you have suddenly developed lock jaw! ... 
Take a breather: Do you know he can hear your nervousness on the phone? ... 
Encourage him to talk about himself: Believe it or not, guys love to talk about themselves too, but ... ... 
Compliment him: Keep complimenting him but do not flatter.