Measures (for example methods, systems and non-transitory computer-readable storage media) to process data in an IMS network, including an HSS, an I-CSCF and an S-CSCF. The I-CSCF transmits a request message identifying a user of the IMS network to the HSS. The I-CSCF receives an answer message from the HSS. The answer message comprises an indication either that the user is assigned and not registered to the S-CSCF, or that the user is registered with the S-CSCF. The I-CSCF performs a predetermined action in response to the answer message indicating that the user is assigned to the S-CSCF.