On the gateway under the field "Call Classification" you can define "OnNet or OffNet". I'm thinking that what you want is OnNet.
Table 36-2 Gateway Configuration Call Classification Settings
Setting Name Description
OffNet
This setting identifies the gateway as being an external gateway. When a call comes in from a gateway that is configured as OffNet, the outside ring gets sent to the destination device.
OnNet
This setting identifies the gateway as being an internal gateway. When a call comes in from a gateway that is configured as OnNet, the inside ring gets sent to the destination device.
Use System Default
This setting uses the Cisco CallManager clusterwide service parameter Call Classification
Larry