The "T" was short for Telephone Man. In this case the number of Telephone Company employee Ray Farrell, who was on The Family's payroll for $5,000 every Xmas Eve. Sonny had Farrell check on Paulie Gatto's and Pete Clemenza's telephone records, so he could tell who betrayed the Don. As we all know it turned out to be Gatto. This was how Sonny found out. Later on page 93, Sonny's tells Michael and Tesio, it was Gatto, and that Clemenza is OK.
