IP PBX systems such as the MX250 can be ordered with a dual T1/E1 interface card. You can deploy in the following way:
Legacy PBXßT1 or E1 ISDN Tie LineÀMX250ßT1 or E1ISDN ÀPSTN
With this approach, the MX250 tie line interface appears to the Legacy PBX as the network. If a DID call comes into the MX250, it can transparently send it through to the Legacy PBX so there is no change in behavior. You can put users on the MX250 and provide them with IP phones. The only change required to the Legacy PBX is a dial plan addition that routes calls to MX250 user extensions over the tie line. With this approach you can evaluate IP telephony and gradually migrate. You can do something similar at a second site if you have one. With this, you can experiment with routing calls over the corporate WAN and network systems together for unified presence, instant messaging, and phone calls. Again, you can then gradually migrate users onto the new system.