Can't connect to dialup and internet

I have a refurbished Dell 760 Desktop with Conexant USB EX93010 ACF modem#2 (COM5) attached. When I try to connect to my BasicISP.net ISP account, to connect to the internet, PC stalls at "Registering your computer on network" and reports error 720. Have double checked ISP settings- they are same as usded on old computer to send this post. Anyone know what the problem is, please?