Great question, good thing I’m quite familiar with VoIP. You are currently using a feature called simultaneous ring, meaning the office app / phone will ring along with your cell phone at the same time causing the issue you are currently having.
You will want to disable simultaneous ring and enable Follow me, the follow me feature will ring your office app / phone for 10 seconds then your cell phone for 10 seconds sequentially. You can use up to 5 different destinations if needed.
You can find more information by tapping or clicking here regarding the follow me feature.