Ring Group calls will not re-poll an extension if the extension became available after the call has already started ringing. That said, you should either use a Call Queue as complex1 mentioned, or, the user can actually pickup the call manually by using the call pick-up feature by dialing *20*RG_EXT where RG_EXT is the Ring Group extension number (example: *20*801)
Note: The default dial-code for call pickup is *20* but yours may be different. You can check what it is under "Settings >> Dial-codes".