Well, you could try it, if you are a VERY, VERY patient person, and your modem connection is ABSOLUTLEY stable.
Setup 3CDAEMON on the PC you are using to dial in, set the FTP directory to where you want the files to be stored.
When you RAS into the S8300, your PC will have an IP address assigned by the S8300 PPP service. This is the IP you will backup to.
Go into the Maint web page, do a "backup now", use FTP, type in the Ip of the RAS connection for your PC, and the login will likely be anonymous/anonymous. It will take a while, be patient.
Mitch
P.S.
A better way to do this is to use a PC the client has onsite, that has either Micrtosofts FTP server running on it, or 3CDAEMON. In fact you should set this up, so it makes daily backups to a customer provided onsite FTP server...