I got the advice to use my electric toothbrush charger from this forum and it works great. I just checked the info on my charger and its rated output is 0.7 to 0.9 volts. I guess if your cell phone charger is similar, it should work.
If you are still using an old fashioned handheld toothbrush...there is a world of difference when you switch to an electric one. A friend of mine at work was singing their praises so I bought one...I would never go back, teeth feel much cleaner to the tongue and a close look in the mirror confirms as much...Plus you get a charger that will safely charge your Zee-Key! What more could you ask for?