With 意思 being いし{HL} (頭高型) and 疎通 pronounced そつう{LHH}, I think I've only heard 意思疎通 pronounced いしそつう{HLLHH}, but since middle-low is not a pitch accent pattern, I'm starting to doubt my ears (which quite frankly fail me more often than they serve).
Also complicating the issue is 意思表示, despite its morphological resemblance to 意思疎通, seems to be いしひょうじ{LHHHLL}.
So it seems the only possible explanation is 意思表示 is taken as a whole word, but 意思疎通 isn't? 意思疎通 does appear quite commonly though. Or am I hearing it wrong?