5.2.2. Changing How Much Time Each Device Rings 
The standard configuration for OpenScape Mobile Pro is to leave a call ringing for a specified amount of time on each device before attempting the next device. By default, these durations are 15 seconds on the Wi-Fi device, 10 seconds on the desk phone, and 15 seconds on the cell phone. Your System Administrator can change the ringing time of a call on each device.
INFO:
This is valid only for Voice-Only and Integrated (Device=Auto-Pilot) configurations.
‣ Parent topic: Section 5.2. More on Receiving Calls