question

Go to user settings, select "call handling and forwarding", and add your mobile number below your office phone number. Set the number of rings you'd like, and if your office phone is not answered, it will forward to your cell. You can also select the option "after hours" at the top to control the hours it forwards to your office and cell phone.

At the bottom, it should state, "Sequentially" under "Phones will ring...", ensuring that they will ring individually, first your office phone for the number of rings you set, then your cell. If you wish for them to ring simultaneously, you can change the setting accordingly.

Be sure to click save when done. Let me know if this helps.