The availability of the (615) area code is nearly depleted, and new phone customers in DeKalb County and other parts of Middle Tennessee are expected to be assigned a new prefix beginning in the spring of 2015.The Tennessee Regulatory Authority voted last week to approve a new (629) area code after the existing numbers have all been assigned. Customers who currently have (615) numbers will not be affected under the plan.The TRA approved what is called an “overlay plan,” meaning that a new prefix area will not created. New customers in the (615) area will simply receive the (629) prefix.The agency said the decision came after a two-month survey, which concluded that 83 percent of Middle Tennesseans favored an overlay to the splitting of the area into two regions.