Hi narkumas,
This not be possible through extension settings. To achieve this you need to create a ring group then add to ring group members the extension which you want to do not disturb from external calls. Next change your inbound rules (that pointed to that extension) to point to the ring group and then you can configure your forwarding rules that suites your requirements.
When the extension is at available status then it will accept calls from the ring group (external numbers). When you want not to be disturbed then you need to switch your extension status to launch where calls come in but the option accept calls from ring groups is disabled.
This way calls will come in the ring group and will be redirected to the destination of the ring group while internal calls will still work.
To change your status directly on you IP Phone you can find dial codes below:
*30 => Available
*31 => Away
*32 => Do Not Disturb
*33 => Lunch
*34 => Business Trip
Also please note if you have IVR and the caller type that extension number, the call will being transferred directly.
