Looking at the N300IP web config I can see that, even after doing a factory reset from the Reboot and Reset menu, it shows four registered handsets. None of these has a device name, the Firmware version for each is set to 0.0 and the Status of each is "unknown". I'm guessing that the N300IP can only have a maximum of 6 handsets registered to it, and that this is the reason why I can only register two of my C430HX handsets at any one time.
The question is, how can I get the N300IP to "forget" the 4 existing handsets that it seems to have registered (probably the original SL56 handsets)?
Apparently, registering a handset that wasn't fully compatible could result in the user being unable to unregister it at a later date. I'm unsure if this was ever fixed or not. Is your N300 running the latest firmware?
Try running the "Firmware Update" option before doing a factory reset of your N300. To factory reset, disconnecting the power supply, hold down the blue registration/paging key and reconnect the power supply but make sure that you hold that blue registration/paging key for at least 15 seconds! You will know when to release, the button will start blinking fast.
Hopefully that will remove the old handsets.