Is there a voicemail app where I can retrieve my iPhone voicemail on my IPad?
Nope. VM systems are fairly closed. There aren't standards/protocols for accessing voicemail in the same manner as there is with email.
Originally Posted by j7469
Only thing I know of is if you use Google voice (free). That way you can use the google voice app.
For those that live in a cave, you can't just download the GV app. It's a service and you need to look into its details to determine if it is suitable for you. It's certainly possible to set up conditional call forwarding so you can use GV voicemail with your existing number.