Definitely don't just take 12v and splice it into the USB power lines.... will definitely ruin something in the computer and possibly the phone.
Does that cable even connect to power connections on the phone? You said it doesn't charge the phone but also said that USB power the phone with 5v..... First see what voltage the phone needs (look at another charger specs or in the manual), and then figure out if the cable has wires that connect to the phone's charger/power connections.
I'm not familiar with that phone but do they make another cable that has a built-in charging port you can plug a power cable into? (Like the motorola cables, etc...) Might be easier and less hassle.