Have MPLS or VPN connections between sites and ensure that these requirements are satisfied:
a. latency is should be less than 100 ms end to end. Figure above 150 ms will produce poor quality voice transmissions
b. VPN router has fast processor and plenty of RAM to handle traffic faster
c. bandwidth should be as high as possible and QoS is set so that VoIP traffic has highest priority. Also, all switches must also be set for QoS for it to work. Otherwise, segregate Phone network from PC network.
d. test the connection for low jitter and packet loss before deploying VoIP solutions
e. g729 will increase 10ms delay by a standard phone’s processor. A fast phone processor would help
f. g729 consumes 20 kbps, so a T1 could theoretically handle 72 calls. One must factor other overhead protocols to realistically assume 80% practical usage of bandwidth to around 57 calls.