Just a thought, but have you tried leaving the numbers ringing for an absurdly long time? I rang someone back the other day and because I was distracted by something on the PC at the same time I let it ring far more than I would usually, and found that there was an answerphone hiding at what must have been around the 25th - 30th ring!

Might also be worth trying at a different time of day, in case the VM is automatically activated in the evening or something?