This backup SIP server (BSS) comprises: means (LMM) for detecting whether an Internet protocol link is not working, and enabling the use of a backup SIP signaling link to the main site via a SIP gateway and a public telephone network when the Internet protocol link is not working; means for transferring SIP signaling information on this backup link; means for, when receiving a registration request from a terminal of the remote site while the Internet protocol link is not working, registering this terminal locally and forwarding the registration request to the main site via the backup link; means (PQM) for storing policies defining what services, supplied by the main SIP server, are compatible with said backup SIP signaling link, and for altering the content of at least one field in each SIP signaling message addressed to the main SIP server before transferring this SIP signaling message on the backup link, this content being altered according to said policies.