Finally I have my answer to my question: no, with one registration 2 people cannot call at the same time, it sounds buzzy, so 1 registration for each phone, and 1 dialplan for each ... I do not understand why Voxalot cannot add this kind of services in their premium offer, I would like to pay for not having to manage 4 accounts, I know, they are very nice to give it for free ...
but ...
