Finally, my old 3011 started to flap with another NIC connected, so the problem seems to be dependent on connected NIC (or its PHY?) as well.Mine is indeed older, factory firmware is 3.27.Our 3011 is old but maybe not as old as yours it was shipping with firmware 3.41
What transport do you use for SIP (UDP, TCP, TLS)?Same on Gigaset S850A. Seems this update breaks SIP on multiple phones... any solution, or is downgrading the only option?
Would try latest 6.42.7 (make sure firmware is updated on System > Routerboard too). Failing that, would write support, linking to this post.