The question is: what cellphones, and what players are available on the cellphones in question? Unless you plan to make the user's download new software just to play your videos, you're going to have to work with whatever is already on the phones in question.
What I'd do is to look at the default configuration of the phones in question and see what players are on it, then contact the developers of that software to find out what DRM they support. For instance, an iPhone by default has iTunes on it, so you'd need to talk to Apple about what DRM is possible, how to get licensed, etc, etc. This approach of using whatever is already on there will give you maximum compatibility with the phones.
Be warned: if your plan is to get the users to download some software of your own, then you have a hard road ahead of you - even with compelling content, you'll find that a lot of people won't be bothered to download new software just to see your stuff.