In Internet Protocol communications based on mobile station number (MSISDN) such as Rich Communication Services (RCS), the operator in a region which should be used to route services to a user device addressed by a mobile station number can be determined by recording which operator is the source of any capabilities exchange or signed presence subscription messages relating to the mobile station number which are received. The identified operator identities can then be stored in association with the respective mobile station numbers in a routing database. Messages to be sent to mobile station numbers can then be routed based on respective stored operator identities associated with the mobile station numbers in the routing database.