I realize that this is some really old information but really needed this in the “cloud” for quick reference. So here it is:-

Overview

Absolute minimum on External Firewall as follows:

UDP 3478 and TCP 443 inbound
UDP 3478 to UDP 3478 outbound
TCP 50000 – 59999 to TCP 443 outbound

References

Determine External A/V Firewall and Port Requirements