One likely issue that's going on is high latency or jitter in the connection. For example, as the packets are being streamed to and from the ISP, some could be arriving much later or earlier than other packets. While most streaming services such as YouTube can handle this as they buffer the connection, with video calls, any packets that arrive out of sequence are effectively dropped as the video needs to be shown in real time, in turn resulting in the picture stuttering.
To test how stable your connection is while it's idle, bring up a command prompt and running: ping -t 220.127.116.11
The ping results should be fairly consistent and stay below 100ms like the following. Make sure there is nothing in the background using the connection, i.e. nothing downloading or uploading. The '-n 100' in my screenshot just means to ping 100 times instead of continuously for '-t'.
I would suggest monitoring this for a minute or two in case there is an intermittent hiccup. If the ping results fluctuate about the place, then it's quite possibly high contention on your ISP or something within your home network using the connection, such as a phone (e.g. I've come across game apps that continuously use the connection in the background).
To stop the ping test and see the results, press Ctrl + C.
The next time you start a video call, run this ping test again with the video call active. If the ping times start jumping about the place, then it's a latency/jitter issue with the connection. While it could be possible the router may be causing this, it's more likely caused by contention within the ISP.
The following is an example with my fixed wireless connection. Basically the area highlighted in red would have resulted in broken up video/voice had I been on a call:
Hello everybody! I use internet mainly for play games and skype and I found this amazing tool when I was trying to understand why I can't use videocall and stream on twitch. Sadly my ISP don't give me the speed that they have to Before TMN I didn't have solid info to claim my ISP because OOKLA (the speed test they use) always lies showing greats speed connection.
Thanks for exist TMN !!!