How do I use my sennheiser HDR 120 with my phone?

those are wireless RF (radio frequency) headphones.

to use them they require a transmitter base (which in this case doubles as a charger) which isnt included with the set you linked me to on amazon.

this means that they are not pairable to your phone by bluetooth. to use them with your phone you would have to plug the transmitter base into your phones 3.5mm connector. provided of course that the transmitter base accepts a 3.5mm input....

you may want to list if you have this transmitter base and what connectors it has.

what it will not do: connect wirelessly to your phone when you are on the go.

what it should do: connect wirelessly to your phone when you have the transmitter plugged in to the phone and the ac socket. (so its not a moveable solution.

----

if you want something that can go anywhere and doesnt have any wires....

you should have bought a bluetooth headphone. smartphones can connect to bluetooth devices without any wires. the only thing wired would be whatever charger the headphones have (and that will be left at home)

a few listed here... http://reviews.cnet.com/best-stereo-bluetooth-headphones/