There's no settings should need changing in your router, evidenced by VoWiFi working.
If it's dropping off after a period of time, I'd suggest troubleshooting that with the MNO - 3 or O2 in this case.
It would be interesting to see if these are the only MNO affected, or whether VF/EE users are also having issues.
I'm a Very Insightful Person, I'm here to share knowledge, I don't work for Virgin Media. Learn more
Have I helped? Click Mark as Helpful Answer or use Kudos to say thanks