Old PBX<--E1-->AS5300<--SIP-->TO OTHER SITES
The Quintum gateways can also be used for this purpose. Once all three PBX systems have the capability of routing calls over IP, you'll need to establish the VPN between the offices so calls can be routed over the private network.
You first need a device at each site capable of setting up a VPN connection. One of the most straight forward approaches is to have the two smaller sites terminate their VPN tunnels to the headquarters site. The hardware requirements at each site are really minimal for this particular purpose since you really just need to have between 1 and 3 VPN tunnels. You can use a SonicWALL appliance (e.g. TZ150) to accomplish this.
The number of calls you can route between sites depends on the speed of the connection. If you are going to use G.711 uncompressed speech, then you need about 87 kbps per call with overhead. If you are going to use G.729 compressed voice, then you need about 24 kbps per call with overhead.
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