I paid my monthly fee on 3rd April but my due date is 8 April,so I am still receiving the message to pay on 8 or they will lock my phone

I paid my monthly fee on 3rd April but my due date is 8 April,so I am still receiving the message to pay on 8 or they will lock my phone.I have the receipts as proof of payment, I went to ackermans yesterday i was told that since I paid earlier my money is gone I have to pay again.