The US Department of Health and Human Services has some literature on that.
According to this page from them, Electromagnetic Pulse (EMP)
Following a Nuclear Detonation:
Although experts have not achieved consensus on expected impacts, generally they believe that the most severe consequence of the pulse would not travel beyond about 2 miles (3.2 km) to 5 miles (8 km) from a ground level 10 KT IND detonation.
Cell phones and handheld radios have relatively small antennas, and if they are not connected to electrical power supplies during the electromagnetic pulse (EMP) they may not be affected, but they do rely on an intact infrastructure for communications.
So basically you'd need to make sure that every single cell tower in the world is hit by a nuke.
I am going out on a tentacle here and assume that there is a strong correlation between the presence of an electric grid and cell phone towers. So if you want to know which areas you need covered by a blast, that's basically everywhere in a world map except for oceans and some major deserts/forests.
In another question I showed some actual calculations made by boffins that show we'd need all the uranium on Earth's crust combined, most of which is unreachable with current tech, to approximate the blast from the dino killing asteroid. That impact had a radius of less than half the surface of the world... If you spread it carefully you could hit all continental areas, but at this point I am thinking that you would need an interstellar economy in order to procure all that uranium. So I am guessing it would cost more than the annual global GDP per year for wvery year you spend on this project.
And that is just considering materials. Your total implementation cost would be much greater due to three factors:
- Labour costs, even if your minions are not unionized,
- Lawsuits from destroying every city in the world as collateral damage, but mostly
- some cell phones connect to satellites, you have to knock those out too.
So there's too many factors to consider that could affect the final cost, but I will say it should approximate to between 0 and 1 Au Bon Pain lawsuits.