IPInstaller,
Each SIP trunk group must have unique IP addresses or Fully Qualified Domain Names (FQDN). If you attempt to create a SIP trunk group with identical IP/FQDN values, an error message appears. This is where you would need to use the Alternate IP/FQDN as some providers use multiple IP addresses to send SIP messages to the 5000 CP. You must add All IP addresses or FQDNs other than the primary IP/FQDN to the list for all calls to be successful.
Hope that helps you out a little bit.
Thanks,
TE