Stab in the dark guess is a bad fax job (misdialed) set to retry endlessly every 15 minutes.
I have a Versalink C405 that wakes up every 20 min or so, when its connected to the phone line(Fax). If i unplug the phone line, then it will wake up only when there is a job to be printed. I have contacted Xerox support, but they concluded its working as intended. I dont think that is correct, and i think they are missing something.
Any help would be greatly appreciated.