Uh oh, this again...
"calling out" can sometimes be more problematic than it's worth. Sometimes it makes sense, sometimes it does not. I recommend using judgement as to how the callee will react rather than a blanket always call out or never call out.
well common sense is always need. It should stand to reason that if you are rolling up on a walker at at 20+ yell isn't going to do crap you'll be on them by the time they hear you. In that case it would be best to just roll on and hope they keep doing what they are doing.