Here are the ports that MSN 7.5 uses for voice/video/file:

Ports 6891-6900 enable File send
Port 6901 is for voice communications
Allows Voice, PC to Phone, Messages, and Full File Transfer capabilities
IN TCP 6891 - 6900
IN TCP 1863
IN UDP 1863
IN UDP 5190
IN UDP 6901
IN TCP 6901
ALSO TCP/UPD 9000-9010