hi guys,
I experimented the same problem using different sip providers. In some case also voice is stopped and only connection is made. I resolved it by using sip operator that works with IVM.
Anyway, now I have another big problem:
IVM answere more call in one time and you can ear the OGM related to the first call in the second one and viceversa. 
This make a stop to all my last month work! because, if i don't solve this, I can not use it!
do you have any solution different than to use it for only one call at time?