When we reported on PictBridge, a new technology in the Sanyo MM-5600, we mentioned that you would need to change a setting to re-enable the ability to use the phone as a modem. However, the released version of the MM-5600 even with the Modem setting enabled cannot be used as a modem.
This appears to be either a hardware or a firmware issue, and not a driver issue. From what we have found, the phone is actively rejecting external #777 calls. It is unknown if this was intentional or not, but Sprint’s track record is to not fix these problems when they surface in phones, as seen in the Nokia PM-6225, and Nokia PM-3205.