10.3 Forward when busy
You can use this call forwarding function to specify whether telephone calls are forwarded when your line is busy, i.e. when you are speaking.
To forward calls when you are already on the phone
1 Open your dashboard.
2 Under
Reachability, tap
Call Forwarding 
.
3 Activate If busy.
4 Enter the destination in the Forward to field. You can choose between:

Selection from the contacts

Entering a phone number

Your voicemail.

Incoming calls are forwarded to the selected destination.