Hi, and welcome to OBi-land.
I recommend that you leave the mobile phone number linked to your Google Voice number. There is no harm in doing so. If you don't want to answer calls on that mobile number, the toggle it off in Google Voice settings. This will preserve your ability to make Google Voice phone calls from your mobile phone.
This also enables you to retrieve your Google Voice voicemail messages from any other phone number. To do so, you need to set a voicemail PIN on the old, "Legacy" Google Voice settings page, on the Voicemail & Text tab, then save the setting.