Core Network for supporting an Internet Protocol Multimedia Subsystem, IMS, enabled User Equipment, the User Equipment lacking Globally Routable User Agent URI, GRUU, support in an Internet Protocol Multimedia Subsystem, IMS, of a communication network, the Core Network comprising an interworking entity, - wherein the User Equipment is configured to send first Session Initiation Protocol, SIP, messages to the interworking entity, the first messages lacking a Globally Routable User Agent URI, GRUU,-related data, and the interworking entity is configured to modify the first messages by adding Globally Routable User Agent URI, GRUU,-related data, - wherein the interworking entity is configured to receive second Session Initiation Protocol, SIP, messages to be sent to the User Equipment, the second messages having a Globally Routable User Agent URI, GRUU,-related data, and the interworking entity is configured to modify the second messages by removing the Globally Routable User Agent URI, GRUU,-related data, -- wherein the interworking entity is: -- a Proxy-Call State Control Function, P-CSCF, entity, or -- a Serving-Call State Control Function, S-CSCF, entity, or -- a Session Border Controller, SBC, characterized in that the Globally Routable User Agent URI, GRUU,-related data comprises a public and a temporary Globally Routable User Agent URI, GRUU, wherein, in case the interworking entity is the Proxy-Call State Control Function, P-CSCF, entity or the Session Border Controller, SBC, the Core Network further comprises a Serving-Call State Control Function, S-CSCF, wherein the Serving-Call State Control Function, S-CSCF, entity is configured to generate a Session Initiation Protocol, SIP, message comprising a binding between - a Globally Routable User Agent URI, GRUU, - a public user id, SIP URI, and - an International Mobile Subscriber Identity, IMSI. - wherein the User Equipment is configured to send first Session Initiation Protocol, SIP, messages to the interworking entity, the first messages lacking a Globally Routable User Agent URI, GRUU,-related data, and the interworking entity is configured to modify the first messages by adding Globally Routable User Agent URI, GRUU,-related data, - wherein the interworking entity is configured to receive second Session Initiation Protocol, SIP, messages to be sent to the User Equipment, the second messages having a Globally Routable User Agent URI, GRUU,-related data, and the interworking entity is configured to modify the second messages by removing the Globally Routable User Agent URI, GRUU,-related data, -- wherein the interworking entity is: -- a Proxy-Call State Control Function, P-CSCF, entity, or -- a Serving-Call State Control Function, S-CSCF, entity, or -- a Session Border Controller, SBC, characterized in that the Globally Routable User Agent URI, GRUU,-related data comprises a public and a temporary Globally Routable User Agent URI, GRUU, wherein, in case the interworking entity is the Proxy-Call State Control Function, P-CSCF, entity or the Session Border Controller, SBC, the Core Network further comprises a Serving-Call State Control Function, S-CSCF, wherein the Serving-Call State Control Function, S-CSCF, entity is configured to generate a Session Initiation Protocol, SIP, message comprising a binding between -- wherein the interworking entity is: -- a Proxy-Call State Control Function, P-CSCF, entity, or -- a Serving-Call State Control Function, S-CSCF, entity, or -- a Session Border Controller, SBC, characterized in that the Globally Routable User Agent URI, GRUU,-related data comprises a public and a temporary Globally Routable User Agent URI, GRUU, wherein, in case the interworking entity is the Proxy-Call State Control Function, P-CSCF, entity or the Session Border Controller, SBC, the Core Network further comprises a Serving-Call State Control Function, S-CSCF, wherein the Serving-Call State Control Function, S-CSCF, entity is configured to generate a Session Initiation Protocol, SIP, message comprising a binding between -- a Proxy-Call State Control Function, P-CSCF, entity, or -- a Serving-Call State Control Function, S-CSCF, entity, or -- a Session Border Controller, SBC, - a Globally Routable User Agent URI, GRUU, - a public user id, SIP URI, and - an International Mobile Subscriber Identity, IMSI.