Systems may establish a Session Initiated Protocol (SIP) session during a Home Subscriber Server (HSS) outage. The systems may include Network Functions (NF) that perform steps for establishing the SIP session and/or bypassing the HSS experiencing the outage. For instance, an Interrogating (I)-Call Session Control Function (CSCF) may modify a terminating message received from a User Equipment (UE) to generate a modified terminating message, and send one or more instances (e.g., forks) of the modified terminating message to one or more candidate Serving (S)-CSCFs. A particular S-CSCF of the candidate S-CSCFs may comprise the registered S-CSCF and may respond to an instance of the modified terminating message. In some embodiments, the I-CSCF may establish the SIP session by sending an instance of the modified terminating message to a designated S-CSCF. The designated S-CSCF may determine which candidate S-CSCF comprises the registered S-CSCF, for instance, by querying the candidate S-CSCFs.