The issue is now fixed, thanks to the help of WofWca. 
For anyone else experiencing this issue that wants to troubleshoot an external relay -
- Create a new profile on both your Primary & Secondary device.
[e.g on phone: “phonetest” on laptop: “laptoptest”] - Keep only the additional relay on the new profiles.
(remove nine.testrun.org) - Then video call yourself from your primary device to your secondary device.
If the call fails, then you know it is that relay that is causing the problems with voice/video calls.