I have been building grouper.ai to integrate voice ai systems with 2600hz's kazoo apis, and have enjoyed doing it as a Greenfield side project. Nerves for hardware on embedded Linux, c/c++ for the voice apis, ports in erlang/elixir to talk the low level calls, using elixir Phoenix in the cloud on the beam. Have used erlang also in day job for custom industrial M2M protocol translation (serial / Ethernet) very successfully also.