Very bad bussines ethics and not professional.

I recently did some work for this client and which was on Tuesday. I installed a MFP printer for them at a client in Midrand. The client signed off all paper work saying he was happy with the work I have done. Now when I asked about my payment for the work done they don't want to pay me at all. I Have send many Masseges and tried calling them as well but they don't respond to me at all. Pls be Very sure if u guys want to do bussines with them. They do CCTV, PABX SYSTEM AND MFP PRINTERS.