I have that problem too. My set up consists of the following:

Main Voipo line and 2 cell phones as simul ring numbers.
When I call the number, it will ring on all lines, but it can only be answered on the cell phones. When I answer from the regular phone that is hooked into the Voipo box, all I get is dead air on both ends. Let me know what your ticket turns up. I am curious and think I have the same problem.