Speaking on the phone is tough because there is a time pressure to say something and the person calling can't see if you are struggling to speak. The first thing I do is let the phone ring a couple times and answer only when I am ready to speak. This takes off some of the pressure because I can control when the conversation will start. I take a deep breath and say hello on the exhale. Another thing I like to do is speak on the phone in front of a mirror (not necessarily for saying hello). It helps me to recognize when I have too much tension in my mouth or my throat, and I can attempt to get out of a block in real time. It also helps me see what other people observe when I stutter in face to face conversations.