How to Forward a Voice Mail on an iPhone
The iPhone's Visual Voicemail service is incapable of forwarding voice mails to another number. Several commercial and jailbroken applications can add the functionality, but they are not required to forward voice mail the old way. To do so, you need to access your basic voice mail by dialing your number and entering in your voice mail passcode. Voice mails can only be forwarded to users on the same network.
Call the voice mail number for your carrier. This is typically your own number. Enter your voice mail passcode to access your account.
Listen to a voice mail message, then press "0," "6" if you are on AT&T or just "6" if you are on Verizon.
Enter a 10-digit number for the voice mail recipient.
Press "#" to confirm and send the voice mail to the user.