Primary WAN link goes down and there is no redundant link at that site. There is no IT staff onsite. An Engineer need to login to the WAN router remotely via a normal PSTN connected to the AUX of the WAN router to debug/troubleshoot as to why it went down. Also, the Engineer needs to access other Cisco device such as Switch stack/PIX to troubleshoot using the same PSTN connected to a modem and/or Access Server.
I had a look at some site and some recommended a PSTN connected to a modem and Cisco AS 2511 which can connect up to 16 devices. But this is now obsolete.