In online dating, how soon do you talk on the phone?

Hi, I met a man online and we have gone back and forth over about 4 short emails...in his last email he said "wanna talk on the phone?" Hmmmm, I am not sure if I want to give him my number this soon, but, perhaps I am being too prude? How can I handle this without offending him? Or, is there a way to share a phone conversation safely at this point in the game? Thanks for your answers.