Is it safe to use a cell phone when I am pregnant?

Yes. The media is "cell phone crazed" now about how many brain tumors you're going to get by calling your aunt celia. There is some evidence that holding a cell phone up to your head for several hours like our teenagers are doing can cause an increase in brain tumor, but judicious use and hands free devices should be fine.
Yes. The hype over tiny amounts of energy used in phones will eventually go the way of the flat earth society although lawyers & gadget sellers will push this for a time. ( they sell energy shields you know) you get more radiation daily from the sun & increase that if you fly during the day. Follow a healthy diet & your doctors advice & you will do as much as anyone to have a healthy baby.