I am not sure that will help you, but you can ask them, since you did ping directly using command prompt (windows) or xterm (linux) it doesn’t matter much, it is an ISP issue.
It is either the ISP doing something about it, which i doubt they will), or you changing service if there is any available in your region.
Edit:
I will ask this here since i had similar problem in the past, but i would take you guys are using wireless router right?
If yes, take a look where it is and what is in the way, i mean between your PCs and the router, i had a LCD TV causing me all kinds of trouble, from packet loss to random disconnections from the router, after changing it to other place it worked fine.
A good way to make a test is, plug the PC through wiring to your ISP modem or ONT (fiber) create a connection using the username and password provided by your ISP (if any, generally there is) if it connect directly, all you have to do is plug it in your PC through the wire and run the ping again, if you get good ping the problem is in your house, either the router or something causing interference, if not, it is indeed your ISP.
One thing ISPs are doing for a while now is providing their own router, VIVO here on Brazil do that and it is a cheap one, the thing is so bad that the DNS pool is 1Kb, when it feel up it start to drop calls for connections from any source, i had an old TP-Link, installed OpenWRT firmware on it and there i have it, can download everything at full speed.
Sorry for the text wall, and what i said above is just to try pin point the issue, not solve it (in part at least).