First of all, astronomy is a field of science which study the celestial object. I believe the word you meant to type is 'astronaut'.
Why can't astronaut use their phone in space?
Because cell phones needs terrestrial towers in order to be able to send messages, receive or make calls etc. The discs on the towers picks up signals from our phone. And then the signals are sent to a satellite in space. No matter how close the cell phone is to the satellite, it will not work. The signals must be transferred to the satellite via the terrestrial towers. The discs on the towers is not able to pick up signal that is above 3000 feet from ground.
That's why, astronauts cannot use their cell phones in space.