[Q] Using a SCh-1535 Verizon S3 on AT&T topic

Hi all, I am using a Verizon samsung galaxy s3 as stated in my title and the question I have is about using this cell phone on AT&T service. I have already added the AT&T apn and changed my network settings to GSM but when I recieve phone calls I no longer have a voicemail box. I downloaded the AT&T voicemail app and it says my number/acccount is invalid. I also reset my voicemailbox pin and tried again and still recieved the same message. I want to know if it is possible for me to get voicemail or if i should just give up on it?
Thanks for your help in advance!