I wouldn’t assume that, since your trace shows that you clearly have a routing problem.
More precisely, it looks like you have a NAT problem. Your internal phones are supposed to connect to eth1 and your external phone are supposed to connect to eth0. I don’t understand what phones you have that are trying to connect through the NAT, since you’ve set everything up to avoid the use of NAT.
The reply is going to 91.x.x.78, which is connected to eth0, but you said that you aren’t using NAT, so there’s no way that’s going to work.
401 unauthorized describes the error you are getting. That might help you with some more information about the failure.