Y cable help

Hey fellow tech heads

I recently purchased a Fritz Box 7390 its great, but the y Splitter cable it came with has 2 RJ11 ports with the 2 pins and the RJ45 connection it leads to only has four is there a way to make a Y splitter that has an RJ12 on one side for the modem and an RJ11 for the phone leading to 6 pins on the RJ45.

I have a crimping and testing set I just need to know if I buy a modular 6 port or 8 port telephone cable could I make one to get better speeds down to my modem, if so does any one know the layout I need for each cap?