Hoe zegt men in het Engels 'zendmast' (meer bepaald die van de mobiele telefonie) ?

Cellphone tower.

Wiki:

Many companies offer to 'hide' cellphone towers in, or as, trees, church towers, flag poles, water tanks and other features.

Mast en tower zijn allebei juist maar hebben wel een technisch verschil:

The terms "mast" and "tower" are often used interchangeably. However, in structural engineering terms, a tower is a self-supporting or cantilevered structure, while a mast is held up by stays or guys. Broadcast engineers in the UK use the same terminology. In US broadcast engineering, a tower is an antenna structure attached to the ground, whereas a mast is a vertical antenna support mounted on some other structure (which itself may be a tower, a building, or a vehicle). Masts (to use the civil engineering terminology) tend to be cheaper to build but require an extended area surrounding them to accommodate the guy wires. Towers are more commonly used in cities where land is in short supply.