Generally speaking it s best to create a task for it. A call off period is a task in itself, and it allows you to adjust periods as and whnen info comes through.

the alternative is to put a lag on the previous task, so if the sequence was order material-- call off period 3 weeks -- arrive on site, you could link "arrive on site" from "order" with a 3 week lag ( xxxfs+15 days) but for me this isnt as good- with a task bar in place, you can see where you are