Well this is dependent on your internet connection speed, too. The codec used is g711 so we know the receiving and transmitting bandwidth is 64k (I believe), but I'm sure voipo doesn't want more than 2 concurrent calls. I have not seen this spelled out in the TOS or anywhere else but I can't think they would like a client using a large number of channels (provided they had the bandwidth) for the low price of $8.25 when a lot of VOIP providers charge additional for each channel.