A method of providing authentication for a Rich Communication Services, RCS, application on a user equipment, UE, for an RCS service provided by an RCS network, wherein the RCS network is separate from an IP Multimedia Subsystem, IMS, and wherein an IMS Session Initiation Protocol, SIP, client is provided on the UE, the method comprising: performing an authentication of the IMS SIP client, wherein the authentication comprises: sending, by the IMS SIP client, a SIP REGISTER request message to a Proxy Call Session Control Function, P-CSCF, of the IMS; and performing, by one of a Serving Call Session Control Function, S-CSCF, of the IMS or a registration service, after receiving the SIP request message from the P-CSCF, an Authentication and Key Agreement, AKA, challenge with the IMS SIP client; characterized by further comprising: providing, by one of a Home Subscriber Server, HSS, of the IMS or a Unified Data Management, UDM, function module, an initial authorization grant for the IMS SIP client, upon successful authentication of the IMS SIP client, wherein the initial authorization grant comprises one of an access token or an authorization code, and wherein the initial authorization grant is provided by said one of the HSS or the UDM function module to one of the S-CSCF or the registration service; providing the initial authorization grant by one of the S-CSCF or the registration service to the IMS SIP client; obtaining, by the RCS application, the initial authorization grant, including one among: i) upon receiving the initial authorization grant by the IMS SIP client, transmitting the initial authorization grant by the IMS SIP client to the RCS application; or ii) transmitting the initial authorization grant by the IMS SIP client to the RCS application in response to a query from the RCS application; and registering using the initial authorization grant by the RCS application, via an RCS Application Programming Interface Gateway, API GW, of the RCS network, for RCS service with the RCS network. sending, by the IMS SIP client, a SIP REGISTER request message to a Proxy Call Session Control Function, P-CSCF, of the IMS; and performing, by one of a Serving Call Session Control Function, S-CSCF, of the IMS or a registration service, after receiving the SIP request message from the P-CSCF, an Authentication and Key Agreement, AKA, challenge with the IMS SIP client; characterized by further comprising: providing, by one of a Home Subscriber Server, HSS, of the IMS or a Unified Data Management, UDM, function module, an initial authorization grant for the IMS SIP client, upon successful authentication of the IMS SIP client, wherein the initial authorization grant comprises one of an access token or an authorization code, and wherein the initial authorization grant is provided by said one of the HSS or the UDM function module to one of the S-CSCF or the registration service; providing the initial authorization grant by one of the S-CSCF or the registration service to the IMS SIP client; obtaining, by the RCS application, the initial authorization grant, including one among: i) upon receiving the initial authorization grant by the IMS SIP client, transmitting the initial authorization grant by the IMS SIP client to the RCS application; or ii) transmitting the initial authorization grant by the IMS SIP client to the RCS application in response to a query from the RCS application; and registering using the initial authorization grant by the RCS application, via an RCS Application Programming Interface Gateway, API GW, of the RCS network, for RCS service with the RCS network. providing, by one of a Home Subscriber Server, HSS, of the IMS or a Unified Data Management, UDM, function module, an initial authorization grant for the IMS SIP client, upon successful authentication of the IMS SIP client, wherein the initial authorization grant comprises one of an access token or an authorization code, and wherein the initial authorization grant is provided by said one of the HSS or the UDM function module to one of the S-CSCF or the registration service; providing the initial authorization grant by one of the S-CSCF or the registration service to the IMS SIP client; obtaining, by the RCS application, the initial authorization grant, including one among: i) upon receiving the initial authorization grant by the IMS SIP client, transmitting the initial authorization grant by the IMS SIP client to the RCS application; or ii) transmitting the initial authorization grant by the IMS SIP client to the RCS application in response to a query from the RCS application; and registering using the initial authorization grant by the RCS application, via an RCS Application Programming Interface Gateway, API GW, of the RCS network, for RCS service with the RCS network.