One such supplement is L-Theanine.
I have a bottle of Theanine of my desk as we speak. I take two tablets with caffeine and in an hour or so I'm feeling extremely relaxed, focused and have a marked increase in mindfulness.
There are a lot of good reviews on Theanine for dealing with both generalized and social anxiety. I don't suffer from it myself, but it definitely helps me focus and puts me in a good mood to make calls.
I can definitely see how something like this could help with call reluctance.
There's a lot of scientific stuff on the Wiki page, but from what I understand is that it raises your dopamine and GABA levels, meaning you become very relaxed in an hour or so after taking them. Because your GABA levels are higher it means that you are less susceptible to stress, caffeine and can focus easier without a constant inner dialogue.
When you are afraid your inner voice is primarily negative, so I can see how this could be especially helpful for newbies.
They aren't cheap though, coming out at nearly 90c per tablet.
The best way I can describe the feeling is when you have had 2-3 beers and are in that sweet spot where you just feel good. It has the same social effect and makes you sleep very soundly at night.
Check it out