you need to manually put your gvoice number in forwarding options. this will change your carrier number to gvoice, but carrier name won't chahne. don't worry gvoicemail will work thou. I assume you are on stock. Also, check the ATT visual voicemail thread somewhere here it was also discussed there.
2013 Nexus 7 FLO 32GB. Rooted.
2013 AT&T Samsung Note II. SkyNote 4.3
2012 AT&T Samsung Note. FJ's ICS (sold)
2012 Nook Tablet. Acclaim's CM7.2. OC. (sold)
2011 Nook Color. Stock Root & OC.
2010 AT&T Samsung Captivate. MIUI 2.3.24 w/Glitch 12b.
2009 T-Mobile MyTouch. BiffMod 2.1. Kept it as a relic.