A method for use by an I-CSCF (12b) in resolving an address of a message according to SIP but using a protocol-agnostic scheme, and corresponding equipment. The method includes: a step (32) of querying a data store (12c) of scheme mappings, in response to receiving such a request, the querying for determining possible mappings to trial schemes at least one of which resolves the address, i.e. which can be used by the I-CSCF (12b) to find the S-CSCF (12e) for the target user (11b) if the target user is an IMS client, or for forwarding to a gateway or next hop in the path to the target user (11b) if the target user (11b) is not an IMS client; and a step (33) of testing in turn (until success or until all possible trial schemes are tested) each possible trial scheme, the testing based on comparing an address using the trial scheme with an address in a data store (12d) of addresses.