The driver converts the RGB to CMYK and sends to the printer.
Some models can print pdf files directly, they can interpret RGB, all printers that can only print a print ready file are CMYK only.
In short, if you LPR a pdf from the command line to your printer and it works, it can interpret RGB, if it fails, it cannot.
Are the Phaser models native RGB or CMYK? e.g. if I send a CMYK file will the Phaser convert it to RGB then back to its own CMYK?