This isn't hypothetical. There is nothing that a radio does that can't be done in other parts of the spectrum. Many FM/AM radios operate in the optical range too. Your TV remote control uses IR. Lasers are used for high bandwidth point to point communications. And don't forget fiber optics, these are all radios that just use optics for the communication path.
Using visible light is possible too, but there is a lot of noise in that band, so it isn't practical unless it is confined to a fiber or a clear channel path.
If you could "see" a FM radio's modulated output in the visible spectrum it would just be a blur of color (appearing mostly white if centered on the visible spectrum) because it modulates colors faster than you could accurately perceive them with your eyes. AM would just look sort of dim like an LED blinking really really fast. See this fiber optic video.
If you want to make your cell phone transmit in the visual range, all you have to do is detect the data (in a receiver) and upconvert the data by AM modulating a LED diode (off and on). If you want FM like modulation, then you have to use something that is easier to shift the frequency of like a fiber optic transmitter.
You can't easily get a piece of metal to emit light like the type of radio you are thinking of in your question, however you can get other structures to easily emit light and it is done all the time.