I have PTSD and am in a dating relationship so I found your post interesting. I think the first thing to remember is that everyone with PTSD is different. There isn't really a cookie cutter than I know of. Withdrawing is definitely something I do. It's sort of like when you have hypothermia, your body uses the blood to keep the organs alive and shuts down other functions. When you are in distress with PTSD, the body focuses on what you have to do and shuts down everything that's not essential (how this is determined is beyond me).

For me communication often goes out the window, I may not be able to talk, or in a very limited way, or avoid things that require 'feeling' or sleep a lot.

Like Kers said though, the inability to respond doesn't mean that that person can't hear you.So phone calls, etc. likely do help even if they are never returned.