A paired unit of radio frequency spectrum (either a band or a block) is treated as one unit, but technically consists of two parts: one range of frequencies for base stations (towers) to transmit to terminals (phones), and a second range of frequencies for terminals to transmit to base stations.
Most cell phone networks used paired bands.
This is also known as FDD.
For example, in the US, all of the "PCS" band is paired. The "A block" of the PCS band consists of 1850 - 1865 MHz paired with 1930 - 1945 MHz. Phones transmit on the lower half, while towers transmit on the upper half.
Paired spectrum is an easy way to keep transmissions in one direction from interfering with transmissions in the other direction.