The best thing really is exposure therapy at least in my case, I just answer the call without thinking because if I think that increases my chance of stuttering. Luckily/unluckily I only really have blocks, so I either have a perfect phonecall or I get a block at the start at some point. The last time I had a block on the phone was a while ago now, calling the doctors which I hate as I have to think about it in advance. I just said 'sorry, i have a bit of a stutter' and carried on.