The port specified in Brosix program settings is for TCP connections – file transfer, screen-sharing etc.
UDP port is randomly chosen with each UDP session (voice or video call) using the machine client dynamic port range. This dynamic port range is usually between 49152 and 65535 on Windows Vista and later, or between 1025 and 5000 on Windows Server 2003 and earlier.
Each Brosix application uses a different port number. The only possibility for interference with a phone system would be if Brosix chooses a port number, which is also chosen by the phone system. In this case a dynamic port map created on the router may not work properly. Since Brosix chooses ports in client dynamic port range (49152-65535), and the phone system uses a port in the range 10000-20000, it is unlikely such port overlap to occur.
Most probably your router/firewall needs to be configured to allow UDP on ports between 49152 and 65535.
Another possible reason is a hardware failure in your router/firewall.