What we call the "light spectrum" is all the human eye can see,
and there isn't much use in being able to go beyond that in either direction.
The range, in frequency terms, is about one octave,
which is why deep untraviolet and deep red look similar.
If you consider the lowest frequency (longest wavelength) we can perceive ---
just above infrared --- as a musical C,
then the highest frequency (shortest wavelength) just short of ultraviolet
is something like the B above that, or possibly even the next C.
As with sound, it's similar but with a slightly different character.
Any radio signals, including those from cell phones,
would simply be distinct colours.
You would not be able to "see" the content of the communications they carried,
although they might visibly pulse if the modulation wasn't too fast (generally 20 Hz and lower).