Buyout firm Platinum Equity LLC is nearing a deal to acquire Securus Technologies Holdings Inc, the second-largest provider of phone services to U.S. prisoners, for close to $1.5 billion, including debt, people familiar with the matter said on Tuesday. The deal illustrates private equity firms’ strong appetite for investments in prison phone service operators because of the strong cash flow they generate from facilitating phone calls, even as they attract criticism over the rates they charge.