EXCEEDING CALL LIMITS

I've been with Nashua Mobile for years (on my sons' phones) and always had a call limit. MTN became our service provider when Nashua Mobile sold their cellphone business. Month 1 saw a bill double the usual compared with when I was with Nashua. Nashua had a call limit and I assumed that MTN would have the same - surely all conditions of contract move across? Month 2 - I phoned the MTN call centre and got through after trying about 6 times. I was promised someone would call me back, which never happened. She did however, re-install the call limit and for Months 2,3,4 everything was under control. Suddenly in April I get another bill for double the usual amount. I feel strongly that MTN should re-credit my account with the difference for these 2 months where the calls exceeded the cut-off limit. The phone belongs to my son and he absolutely will phone if there's free airtime. In Nashua's days, they would just lock it so that he could only have limited call usage. Can't MTN do the same consistently? I am now complaining on Hello Peter, as I did give MTN a chance to handle the situation and it wasn't handled.