bruno,
according to lcr/README file
A gateway matches a request if user part of Request-URI and "From" URI match the prefix and From pattern of the gateway. Matching gateways are then ordered for forwarding purpose (1) according to longest user part match, (2) according to gateway's priority, and (3) randomly.
so if you have one lcr entry with sip:123@.* as from_uri regexp and another lcr entry with empty from_uri, and call comes from 123, then both of these entries match if their prefixes match and the one with longest prefix will be selected of if same length, one with priority.
i'll add a note about empty from_uri entry into README.
-- juha