fix a pc internet problem

Small pet grooming shop in Long Beach. One Windows XP computer. Frontier FiOS internet. Some internet connected devices are not working: one VoIP phone (but the other one is working), one Verifone credit card terminal; the alarm system. We are able to access the internet on the PC. Also, the wireless dropcam is working normally. There is a Netgear VPN box and an old Verizon box; maybe one of those boxes is causing the problem? I did not set up the system. I don't know what functions the Netgear and Verizon boxes are performing so I don't know how to fix the problem.