Since '9' is the default prefix of out-group calls, we need change the out-group prefix firstly.
Step 1: Please refer to this topic to change the default out-group call prefix.
Step 2: Please click menu "Dial plan / Analyze called number" and add a new record with following values:
Called number prefix = 9
Route type = local user
Then, when we dial "9xxxx", calls will be sent to extensions "9xxxx" directly.