You can make a rule on your extension - example when busy forward to an IVR.
Create a prompt and upload this to the IVr - Example - Hi this is the automated attendant of extension 100 Rick James
Press 1 for Voicemail, 2 for X, 9 to redirect to my mobile.
Option 1 will be sent to the voicemail of extension 100
Option 9 will be forwarded to an external number (outbound rule + mobile number)