Dave:[color=blue]
>There must be a number - I have two different incoming to the same IP
>on the same port[/color]

That's because they have registered themselves with your VoIP provider and
the SIP protocol can differentiate
between them.

If you dial directly the IP number of a router, it doesn't need any user id
/ no, and would connect to a SIP device on port 5060. If there two then one
or both should ring.
Regards,
Martin