Third party devices were "legalized" by the Carterfone decision in the 1950s, long before the breakup. https://arstechnica.com/tech-policy/2017/12/carterfone-4

Third party devices were "legalized" by the Carterfone decision in the 1950s, long before the breakup. https://arstechnica.com/tech-policy/2017/12/carterfone-40-years/ https://arstechnica.com/tech-policy/2017/12/carterfone-40-ye...