Nimble group and Mtn service providers very ***********

The agents of this company keeps on calling me saying i have an outstanding balance for mtn account i keep on indicating to them that the account was paid in full and i even sent the proof of payment in their email, *** and in their WhatsApp group as advised by one of their agents but they dnt do anything about it ,all they do is to keep on calling me they dnt check their email now i am still classifed as non paying persom because they're failing to do their job