Darrel,
It would obviously only work in SIP mode so you would need a SIP license on the 5000 and they would need to have a Public IP address for the Base Server NIC and a different one the PEC-1 NIC [if they have one]. Then they would just need to setup a firewall rule that open up port 5060 from that Public IP Address to the LAN IP Address(es) of the 5000.
Now, it will not have the same feature set that it would if it were connected to a Cisco Phone System as we do not necessarily support all of the same SIP features as the Cisco SPA 504G can support with the Cisco Phone System.
Thanks,
TE