The phone line lies outside your house. If lightning strikes, the strike will travel to every phone on the line and potentially to you if you are holding the phone. So, if you are indoors during a lightning storm, stay off the phone. If you must call someone, use a cordless phone -- that way, you're not in contact with any wires that run outdoors.
Cordless or Mobile phones are low power devices and do not have any characteristics which would make them attractive to lightning strikes. So chance of striking of lightining is very less.
So, Do not use corded phones. Cellular or cordless phones are okay.