Where is my airtime

I have had an MTN contract for years and when I run out of airtime I have to buy my own. I bought airtime last week Monday and it was not credited on my phone but debited the amount from my account. I tried phoning MTN for days several times with no luck and when I finally got through I was told I must phone my service provider Nashua Mobile. I have never been to Nashua Mobile, I upgraded this phone at an MTN shop. MTN am I really going to get my airtime money or what. I also never knew that when I go to sign a contract at MTN I actually signed with another company, in that case I might as well cancel the stupid contract then.