When you port a phone number from one carrier to another carrier, the new carrier tells the organization that controls phone number call routing, NPAC, to point inbound calls for that number to the new carrier's phone switch. The number is completely removed from the old carrier, and so that service is shut off. The SIM becomes permanently disabled and can be thrown away (however, the phone handset can be re-used with a new SIM).
After this is completed, you have a Google Voice account with an inbound telephone number. There is no forwarding phone number at this point. Now, you can add and verify some other phone number (i.e. a different cell phone number) as your forwarding phone. Google Voice treats "Chat" as a forwarding destination, and OBi devices are simply acting as Chat clients.
There are two ways to record a greeting: you can add a forwarding phone number, and have GV call that number, or you can call your own GV number from the OBi-attached phone. When the Google Voice voicemail system answers, you can get into its administrative menus, and select 4, then 1, to record your greeting.
If you add a forwarding phone number, it will enable you to use GV to its full potential, forwarding calls to that phone, checking and listening to voicemail from elsewhere, etc. Be sure to set an 8-digit voicemail PIN on your GV account, to protect your mailbox.