USB Sennheiser Headphones

I have a Sennheiser M145 USB headset which I use on my mac for listening to music and chatting on Skype etc. The wire from the headset is connected to 2 x 3.5mm plugs, one for the mic input and one for the headphone output. Then an adaptor attaches to these 2 plugs and turns it into a USB connection.

What I'd like to know is how can I use this headset on my iPhone? I have found Belkin Y splitters, but I dont think this is what I'm looking for... them seem to be just for speakers