neither safe nor legal. Look at the terms and conditions of your phone contract. Operating aeronautical mobile is absolutely forbidden.
See also the federal law on the matter
47 C.F.R. § 22.925 Prohibition on airborne operation of cellular telephones.
Title 47 - Telecommunication
Title 47: Telecommunication
PART 22—PUBLIC MOBILE SERVICES
Subpart H—Cellular Radiotelephone Service