No sicario666, that's not correct. I have it forward to my cell and home numbers. when I call from my either of those, I get my gv voicemail recording. I press * and then my pin, and THEN it gives me my messages or lets me make a call. You might have to dig through the settings on voice.google.com to make this all work. The gv app doesn't have all settings, and I don't think the mobile site does either.
Also, if you want to ONLY use it for voicemail, you need to dial *71xxxyyyzzzz from your cell. The letters are your 10-digit gv phone number. This lets incoming calls ring your phone, but forwards missed calls to gv. In this case, you would turn on gv's "do not disturb" feature so that it doesn't ring a few more times before voicemail picks up.
Good luck!