In the early days, they were ubiquitous, and a necessity. As networks have switched over to G2 and now G3 technology, they're no longer a necessity. However, there are after-market companies making devices for handhelds to "drop into" which are connected to an outside antenna. They amount to a passive repeater, and are more hype than useful.
There are also stubby, glass-mounted antennas for both amateur and commercial frequencies. They're about as useless as the aforementioned.
You could be seeing either one or other similar ones, and assuming they're cellphone antennas, when they're not. Easy mistake too, as they look very similar.