Thanks for reaching out!
Considering you are experiencing a similar issue while on a different network using the same computer I would guess that the issue is originating from the computer itself. This could either be related to the Network Interface Card/Wifi Card or how the program itself is running on your machine.
I suggest running a trace to your loopback address (127.0.0.1) from inside PingPlotter to test your Network Interface Card (NIC). If you are seeing packet loss to the loopback address this is an indication that the NIC card in your machine is not working as intended.
Here is an article on Solving Hardware Problems
With that said it is also a good idea to run a trace to an external target such as the server you are connecting to for your VOIP phone. If you can not find the IP for that another good target would be something like the Google DNS server (18.104.22.168).
To learn more about interpreting your PingPlotter results I suggest looking at the following articles: Common Network Problems Interpreting PingPlotter Results Interpreting Latency and Packet Loss
If you have questions about your PingPlotter results feel free to share them with us after collecting at least 24 hours of data by:Sending over a SampleSet(pp2) file
(File -> Export Sample Set -> All Data)
Hopefully, this helps!
Best of luck,