You can setup the service parameter such as National number prefix with a 9 for MGCP gateways. The draw back with MGCP gateways is that you can set either 9 or 91, but not both. In this case its safe to set the prefix as 91.

With H323 gateways you can set translation profile to match the area code and prefix a 9 or 91 in front of the calling number.

HTH

Sankar

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