Gtalk2Voip has some glitches.
Have them try calling
123456@us.voxalot.com as well as
*0101234456@sipbroker.com and see if you have better results with one over the other. (where 123456 is your voxalot number)
Another thing to do is try the call via GTalk2Voip while logged in GoogleTalk rather than MSN....
If any of the above succeds, the problem is likely on GTalk2Voip's side....
If not, we are back to square one...
Also how many seconds do you have it set to voicemail...try 20+