Unpack initramfs.cpio.gz (boot.img) from MTK6589 phone?
I also had problem extracting initramfs.cpio.gz , until I opened this file in editor and seen first 3 ascii letters  (the header) in my case it was "LZO" , so I found proper soft to decompress 
"lzop -x " you can find it probably in every linux, for example knoppix has it

maybe you will notice other header in your file , so you will need to google it or have a good guess

good luck