I have same issue with X-lite. Tried various settings without any luck accessing voicemail as well.
Edit: After Nate425
posted, I searched the web again and found this from Counterpath regarding the DTMF settings. This may not provide the exact solution for the OP, but may provide some direction. In the X-Lite 3.0 softphone, there weren't any options for DTMF settings in the GUI. After searching the web, I found that there was a code that I had to dial and send in order to access a more advanced option set. Here is the posting I found from Couterpath Customer Support that now makes calling voicemail work for me:
"Here are the instructions on accessing DTMF parameters and changing for SIP INFO DTMF:
Dial ***7469 (send)
Once you get the advanced options menu up, filter first for DTMF. Find the Force Inband DTMF entry, and change the value to 0.
Then filter for 2833. Find the RTP 2833 enabled line, and change this value to 0.
This will work for servers requiring SIP INFO DTMF. If you are still not successful, please try any combination of enabled/disabled until you get the setup that will work for your server.
I hope that helps
_________________
CounterPath Support"
Edit: One additional thing - I also had to make sure I pressed and held down each number of my password long enough to be recognized. Before making the above change, nothing would get vmail to recognize my password (holding down the keys longer, etc). After the change, pressing the keys too quickly still resulted in not recognizing the password. Holding each key down for just a little bit longer did the trick (maybe there is another setting that would help with this as well).