You can access your iPhone voicemail from any phone by dialing your iPhone's number from the other line. After calling your number, to access your voicemail:
On AT&T:
Interrupt the voicemail greeting by pressing the star key (*).
Enter your password (the default password is your 7-digit phone number without the area code).
On Verizon:
Interrupt the voicemail greeting by pressing the pound key (#).
Enter your password followed by the pound key (#).