try this bai. dunno exactly if mo work ba.
just by using your pldt plus sim card, instead plugging into your mobile phone a device called gsm gateway (dont know tha exact term for this device), this device can be used using your analog telephone basically its an ATA(ANALOG TELEPHONE Adopter) device for sim card module. one connection called an FXS port( foreign exchange station) this port has an rj 11 connector an output port like a regular telecord that can be plug to your existing fax machine.

this works like magic. hehe