Absolutely frustrated

I'm highly disappointed in durban north post office as for the past 3 weeks I've been trying to call them on their land line plus manager office line and they dont answer if I could screen shot and post a pic of how many calls i make daily at different times with no answer, on monday I went in to the post office and requested they get my letter that has been sent internationally I've been tracking for months , it was sent from Umgeni station mail center to durban north post office on the 12th of April. Now the mail center and post office do not know where it is they are of no help , I am spending money running from pillar to post as they send us around to go look. I need to know how to take this up or who to speak to for assistance cos now I'm at frustrated.