Can't call computer to computer pls help

Hi, please help me w/ my problem. I can’t establish a call between two spark account. Here is the message “session closed due to closed remotely” and sometimes "session closed due to closed remotely. This maybe caused by firewall configuration problem " I checked the firewall but its turn off. But when I call within a network I can established a call. Please help me guys. Thanks in advance.

Our network:

192.168.0.0/24 10.100.0.0/24

spark =====================>openfire<========================spark

Windows server2k3