1. Public IP forwarded to IP of PBX.
2. Open UDP 5060 (SIP Signaling) for inbound and outbound traffic
3. Open UDP 49152-53246 (RTP Traffic) for inbound and outbound traffic
http://www.ipofficeassistance.com/sip-flowroute-to-ip-office-r9/
http://web.archive.org/web/20120106133717/http://www.ipofficehelp.com/HOWTO_SIP_Flowroute_R5.html
http://www.ipofficeassistance.com/tag/sip/
https://business.timewarnercable.com/content/dam/business/pdfs/services/Config%20Guides/Avaya_IP_Office_9.1_SIP_Trunk_Config_Guide.pdf