Question: how much does it cost to add another phone on at&t?
Answer: Like most carriers, AT&T charges you for the data package, plus a device connection fee for each line. Below are the monthly prices for up to four lines on AT&T's Mobile Share Plus plans, not including taxes, fees and discounts. 3GB: $60 for one line; $100 for two lines; $120 for three lines; $140 for four lines.