Poor service

I have been a loyal customer for almost 10 years and I have never had a problem with MTN until now. I have paid my account every month on time(1st of every month) and now I am suddenly in arrears. According to MTN I must pay my account within 21 days of my invoice date. According to my statements that was sent to me by mtn my invoice date differs from the 16th to the 20th, which falls in my 21 day period. You change invoice dates as you please and is now requesting extra payment from me like it is my fault? Please check your records. Please clarify as to when I went in "arrears" and please rectify this as the fault is on your side not mine.

I am also being threatened that I am going to be disconnected.