change into active voice

This sentence is passive for two main reasons:

1. The main verb is written in the past participle ('used')
2. You have used the auxiliary verb ‘be’, which is a passive 'state-of-being.'  

Remove the passive phrasal verb ('be used') and replace with an active verb ('to use').

Here are some options.

A: 1

"While driving, do not use mobile phones."

A: 2 (add a character for clarity; although, some flow is lost).

"Drivers should not use their mobile phones while driving.” (Subject: drivers; direct object: mobile phones).

A: 3

"Do not use mobile phones while driving."  (imperative, yet passive).

Passive is correct? It is not always necessary to change everything into the active voice. I actually think the passive works best in this example. We don't notice the missing character/s--these are the people using their phones, and driving their cars--because the agent of the action is obvious.

You don't need to change a passive:

1. "If the agent of an action is self-evident" (Williams, 65).
2. "If it lets you replace a long subject with a short one."
3. "If it gives your readers a coherent sequence of subjects."
4. If you deliberately want to hide the subject of an action.
5. If the passive verb "helps your reader move smoothly from one sentence to the next" (54).
6. "Mobile phones should not be used while driving."

Hope that helps!

-- Iain

Willams, Joseph M. Style: Lessons in Clarity and Grace. http://goo.gl/WHC6t (brilliant book)