Though the C139's design is basic and functional, it does have a bit of style. We like the basic black color scheme and the rounded corners that give it an aerodynamic look. While compact and easily portable (4.0 by 1.8 by 0.9 inches), it also weighs a bit more than we thought (3 ounces), which gives it a solid and comfortable feel in the hand. Other good points include the rubberized sidings that line each spine and the absence of an external antenna.
The C139's display is small at 1.25 inches diagonally, but it's nonetheless bright and colorful, with support for 65,000 hues. You can change the backlighting time and the contrast, and though the menu font may be a bit small for some users, the dialing font is rather large. And speaking of menus, it's worth noting that the C139's interface is simple and utilitarian. Beyond the simple animation, don't expect any flashy graphics or a choice of menu styles. You can, however, change the order of the menu choices.
Our only real complaint concerns the navigation array. Though the controls, which include a five-way toggle, two soft keys, and the talk and end/power buttons, are tactile and well-sized for the C139's small form factor, an annoying quirk made them less intuitive. For example, while the button in the middle of the toggle opens the main menu, you must then shift your finger up to the left soft key to select certain menu items. Though we got used to the constant finger switching eventually, the reasoning behind such a design choice baffled us. We think it would be easier if the menu button doubled as the OK key (as is the case on many cell phones). Still, we like that the toggle is raised above the surface of the phone and that it's covered in a rubberized material. Only the two soft keys and each numeric button can be set as user-defined shortcuts while the up and down directions on the toggle open the phone book automatically. There's no dedicated back button, but the right soft key serves that purpose when in a menu.
The backlight keypad buttons are easy to use; as well as being large, they're covered in rubbery material and raised above the surface of the phone. The only other features on the exterior of the C139 are a headset jack on the left spine and a charger port on the bottom end. Since there's no volume rocker on the C139's spine, you have to remove the phone from your face and change the volume via the navigation toggle.