nba attorneys

I keep getting calls about a Nashua Mobile phone contract that i has back in 2011, of which was handed over to MBA ATTORNEYS i have settled the payment with MBA in 2015 but keep getting pestered by NBA ATTORNEYS i would send my proof of payment and they keep saying we will escalate the matter but nobody ever gets back to me they will skip a month or so then someone else will call regarding the same issue. How many times more must i send them my proof of settlement payment . This is very frustrating