source: The Republican (Danville, Indiana) - issue of Thursday, June 27, 1957 - page 1, columns 6-7
According to a June newspaper article, on August 4 at 2:01 am, the towns of Plainfield and Brownsburg were to go live with direct dialing of telephone calls, using new phones that had rotary dials. Before that, if a resident wanted to make a call, they had to lift up the receiver and ask the operator to complete the call. As with when Danville got direct dial service in 1954, Plainfield and Brownsburg got phone numbers that started with two letters and then had 5 numbers. The 2-letter prefix was given a name to make it easier to remember - Danville's prefix was SH ("Sherwood"); Plainfield's prefix was TE ("Terrace") and Brownsburg's prefix was UL ("Ulrick").
By having Plainfield and Brownsburg join the Indiana Bell telephone exchange, calls made between Danville, Plainfield and Brownsburg were now local calls - previously they were long-distance calls, subject to additional charges beyond the basic monthly rate.