Wilson Electronics
Cell Phone Antennas and Cell Phone Boosters at Wilson Electronics is the place to look, They make all sorts of antennas, and adapters for just about every phone, EVEN if your phone has no antenna jack they have an adapter for it.
Another options, which they also make, is a cell hpone repeater.. here is how they work
In my case I put an antenna, actually on the back of the rig (It could be roof mounted but why poke a hole in a perfectly good roof if you don't have to.. I poked the holes in the rear cap just below the roof line, the antenna sticks up over the roof) The cable then comes in via the rear cap, then in via the real rear wall of the motor home (So it's weather protected at the entry point) to a cabinet in the rear most portion of the motor home.. There sits a Wilson bi-directional amplifier... Up near the front of my 37' motor home, over 25 feet forward, is a 2nd antenna, hidden in a cabinet..
The cell phone needs only be close to the 2nd antenna, no wires needed.. The chair my wife sits in is next to the antenna.