I cannot call 173.what is going on?

Hi. <br> I ported my number***) two weeks ago from telkom mobile to Mtn. <br> now i can call any number beside 173.i have been at mtn store waterfront/capetown and done sim swap but still i cannot call.when i call 173 line drops immediately. i called 173 from different number call center give me some ussd code to remove call baring but still same. I CANNOT CALL 173.now guys listen if you don't want me as a customer just let me know. if you guys waste my time between call center to mtn store no ways. i have spent enough time to fix this problem and i am frustrated. please sort this problem or i ll port my number to different network. regards