[App_rpt-users] one-way connection making

I have two nodes, behind a common router, that are quite similar, except that - on the internet side one appears as port 4569 - and the other as port 4568. For the longest time, I have found that I can connect only in one direction. In the other direction I get the "chan_iax2.c:4071 __auto_congest: Auto-congesting call due to slow response" failure message. As it happens, the one I can do the connecting connect with is the one that uses the 4569 port. I would expect that I can connect in either direction. What can cause this one-way problem? Ken