In most cases, the current WAN link will need to be upgraded in order to support converged voice traffic. The question that must be answered is how much bandwidth is required for each WAN circuit, and this question cannot readily be answered without significant analysis of the current voice call patterns and volumes, as well as future call patterns and volumes using VoIP.
Bandwidth for voice: A deeper look
Traditional voice calls using Time Division Multiplexing (TDM) technology use 64 Kb per call. VoIP uses IP as the technology to enable transport of voice traffic over a data network. The voice data is encapsulated within an IP packet. The process of putting the voice data into the IP packet is called encoding. The method used to encode the traffic and the transport mechanism (ATM, Frame, Ethernet, etc.) can have a great impact on the size of the packet. Two common options for VoIP packet encapsulation are G.711 and G.729.