Hi:
Google Voice is an inbound call forwarding service. When someone calls your Google Voice number, it will simultaneously forward the call to all of the verified, enabled linked phone numbers (up to six numbers) on your Google Voice settings page. It will also forward to all OBiTALK devices you've configured to use that account, and to any Hangouts clients you've set to ring on inbound calls.
If you don't want the mobile number to receive those forwarded calls, you can delete it. However, having at least one 10-digit number is essential, if you want to access your Google Voice voicemail over the phone when you're away from the OBi. Google Voice uses the linked/forwarding number as a means of authentication to access voicemail.
What's your motivation to delete the mobile number? If it actually belongs to someone else, then delete it. If you plan to keep active/paid telephone service on that number, then don't delete it.