The modems on the 5000 are bad.... and really slow, they essentially make a PPP ethernet connection and flow the exact same information as a local LAN connection. If you are going to use it, make sure to set the minimum baud rate to 2400 (or as low as it can go), but it is still better than a 2 hour drive to change a hunt group or something stupid like that.
Still, the best bet is to make it part of the installation requirements that some type of high speed access is required remotely, whether that be by VPN, LogMeIn, Bomgar, direct TCP ports, or installing the software on a customer PC and manually building a session each time with something like ShowMyPC.