For VOIP, first you need to consider the codec to use. Although there are many voice codecs, MyCo wants the safest bet and suggests that you design with the most known and oldest codec G.711 since they are very concerned about the voice quality with VoIP. Calculate the voice payload from G.711 codecs with 20ms. frames along with the RTP, UDP, IP, and Ethernet overhead to measure the bandwidth per call with G.711.