The ESI Hosted Services SIP trunks deliver calls to your PBX on the standard SIP port number 5060 on UDP.

If your PBX sits behind a firewall/router (the most likely case) and has an internal NAT address of 192.168.xxx.xxx, 10.xxx.xxx.xxx, or 172.16.xxx.xxx, you will need to make sure you have port 5060 UDP forwarded in your firewall/router to your PBX's internal NAT address.  Most firewall/routers support port forwarding.  See your firewall/router's documentation for information on how to setup port forwarding.