(Got this one Richard get back to that custom grouping of shapes you're working on!)
Lou, you can absolutely 'map' all your cell towers (even if they are in the middle of the ocean!) To make it easy, call your custom lats/longs 'Latitude' and 'Longitude' then when you connect to the data source Tableau will automatically give them the proper geographic roles. If you have a tower ID or some other unique identifier (name, whatever) then just drop that on the Level Of Detail shelf, and Tableau will plot all your tower. There's lot more you can do, but I won't go on...
Rest assured Tableau can handle your mapping needs (mostly).