The latest update to the rest apps has fixed the connection issues.
Now from the aastra 6730i I can get the voicemail menu and scroll though the messages, if you press enter on a message then it pauses, then returns to the voicemail menu.
The phones have enough keys to make it work, as demonstrated by the free aastra version that we had on our old server.
The issue with this paid for version is that when you go down you list of voicemails, click to view, you do not get the play / delete options you get returned to the voicemail menu!