Auto attendant is an OBi thing, and voice mail is not.
I think you are trying to access GV voicemail by dialing from its associated phone. So when you say it just rings, does the resulting incoming call show up in the OBi202 call history? Look at the From field, and note if it is coming in to SP2, SP3, or SP4. Maybe the numbers are not associated with the SP you are thinking.
After you get this sorted out, I suggest you put each of the numbers with the associated prefix as speed dials via OBiTalk. There would be an alternative way, but the speed dial will be easiest all around.