Deleted voice mail

Solution
Unfortunately, voice mails aren't stored on the phone. They are through your provider. You can check with them to see if they allow for undeleting messages. I know some do have that option.

Now if you are talking a text version of the message, that is different. It will depend on the program you use for the voice mail text messages. Some have trash, like a computer, that you can get them from, some don't. You would have to check the individual program.
Unfortunately, voice mails aren't stored on the phone. They are through your provider. You can check with them to see if they allow for undeleting messages. I know some do have that option.

Now if you are talking a text version of the message, that is different. It will depend on the program you use for the voice mail text messages. Some have trash, like a computer, that you can get them from, some don't. You would have to check the individual program.
 
Solution
Every company handles it differently. Some will tell you they can do it, some won't. It is really up to them. Often even if they have them, they may not say they do/can. Your only option would be to try again and maybe speak to a supervisor, but there is no guarantee on getting them back. Ever.