Is call wait set to yes in the did?
Is there a max call entry?

Other suggestion is since she's the only member, send the did direct to her extension and adjust the vm.

The settings tech support are talking about won't do anything in this case, they're to-do with the svmi20i using the back plane (tdm mode), and just like you change mgi ports if you have multiple card and nat, the same applies for public svmi and handsets