I tried the app several times and found it a frustrating experience. First, you click on 'send mail' and the app brings up a list of your contacts. You are still within the VoiceMail app, and if you have a long contact list, all you can do is scroll up or down. No shortcuts. You select the contact, and are instructed to add a short line of text. I entered 16 words, and got an error telling me the limit is 25 words. Grrrrrrr. I shortened my message, then proceeded to record a voice message. After hitting the' send' button I waited for it to go out. Then I waited some more. I noticed my network signal was dipping below 2 bars, so decided to quit. Unfortunately, there was no option to cancel. I forced quit the application and went back in. No sign of that message, so I had to start from scratch.
By this time, I was in a better signal area (4 bars, solid 3G) so I sent another message. This one succeeded, but it took 24 seconds to send a 4 second message. I don't think this app is designed for long, detailed voicemails. Maybe on Wi-Fi.
For further testing, I sent myself a message. It arrived in my mailbox, and it said if I wasn't on an iPhone to click on a URL to retrieve it. Well, I WAS on an iPhone, and saw no indication that there was an attachment to click on or listen to. There is an in box on the app, so I checked there, and hit the refresh button. No message there either. In desperation, I clicked on the URL and got a web page with no way to listen to a message. Grrrrr.