Complex sentence: Mobile phone services do not use call signs on-air for obvious reasons (the phones and their users are not licensed, instead the cell operator is); however, the U.S. still assigns a call sign to each mobile-phone spectrum license.
Split into simpler sentences: Mobile phone services do not use call signs on-air because the phones and their users are not licensed, instead the cell operator is the one holding the license. However, the U.S. still assigns a call sign to each mobile-phone spectrum license.