American Tower Corp. has agreed to pay about $182 million for 2,100 towers in Colombia as it seeks to improve its economies of scale in the South American country. The seller is Millicom International Cellular SA. Plans call for a Millicom subsidiary to sell the towers to a new Colombian subsidiary of American Tower. The towers are expected to be transferred in the fourth quarter. Boston-based American Tower owns, operates, and develops broadcast and wireless communications sites.