Iax won't connect

hi
i have a dual asterisk server connected through an iax trunk, one server (A) with a fixed public ip address and the other (B) dynamic ip.
the iax trunk wont connect unless I add a new ip route to the A server
ip route add ip(A) via 192.168.1.1 dev eth0

server B--------router----------------------------------------------------serverA

does anyone knows why?
thx in advance