Request for assistance

I have an MTN SYM card which I has been OK. Last week, I bought airtime and every time I try to load, I get rejected with the message "Pin Number invalid" I cannot get help on the 135 Help line number as none of the options are relevant to my problem. I tried 136 and the message I got was that something is wrong with my number and it will be corrected. To this day, I cannot use my phone and there is no other way I get get help from MTN except through the electronic route. How can I resolve my frustration? Regards