A decision to stop a benefit arrives as a letter. Somewhere near the end of it there is a line about objecting within six weeks.
The six weeks run from the date on the decision, not from the day it reached you, and they do not pause while you try to reach someone by telephone. A short objection filed in time can always be supplemented later; a full one filed late usually cannot be heard at all.
If the letter has a date on it and you disagree with it, the first useful step is to put the objection in. Everything else can follow.