John, that scenario required the creation of a SIP trunk, but each FXS port can still register individually. Username and password are configured per user, but the proxy and server information is configured per trunk. The FXS user would look like this:
voice user 5551000
connect fxs 0/1
sip-identity 5551000 T01 register auth-name 5551000 password 1234
In the above example, T01 would contain the SIP server information. You could have multiple SIP trunks with different servers if the users weren't using the same one.