Have the same problem. For answering the phone, I found that clearing my throat before I say "Hello" helps a lot. When I'm just talking, I've learned to sort of feel when the block is coming and pick a different word. For whatever reason, if I choose another word rather than the one I wanted to say I can usually talk around it. Do you have any words that you know you can always say? "Well" or even "Uhh." Anything just to get over the hump.