matching for ingress session-agent with hostname

So when a session-agent is configured like the following:

session-agent
               hostname fqdn.fqdn.net
               ip-address
               port 5060

How does the SD match this in an incoming Invite that only contains IP-addresses?

Does it do a reverse lookup? If so in what cases does it do a reverse lookup? Does it look in a header somewhere in the Invite?

I have noticed that whenever the dns cache-entry is empty (ie no calls towards this agent has been made) no incoming calls will be allowed either.

What would be the best (correct) way so fix this?

The reason for this session-agent is that the remote end (ie fqdn.fqdn.net) wants to be able to update the record (to change loadbalancing etc) without involving my side.