BP Amoco plans to post signs warning customers not to use cellular telephones near gasoline pumps at its U.S. stations by the end of the year. The precautionary measure is prompted by concerns that electronic impulses from a cellular phone could help ignite fires if gasoline or gas fumes are present. The signs at the Amoco and BP stations will have a picture of a cell phone enclosed in a red circle with a slash through it, [Linda McCray, a BP spokeswoman] said.