Unfair billing

I put my 30 days notice on the 1st if June for a contract that was ending on the 2nd of July 2020. The number has bee deactivated t cell c has billed me for the full month of July even thou the number is no longer active. Everytime i call cell c each consultant give me a different story as to why i was billed. On the 7th of this month, the consultant i spoke to said they will escalate the matter i will get feedback within 2 day none of dat happened. Now im being told they cannot change my bill.