I am looking at converting my office of 20 people into a VoIP system using asterix. The software configuration and setup seems OK. What I am curious about is the hardware needed. More specifically, what type of connection do I need to the telco to have one phone number to reach the helpdesk pool (4 people) and a couple direct lines (no extension needed). I would think I’d need one BRI for every 2 phone numbers. What kinds of connections do you think I’d need? If someone is using one line, can other people use it or will it be used up as an analog line would be? I’m probably not making sense, I’m in the learning phase. I appreaciate your input.
Or you can just do everything over the internet buying a quarter or half T1. And then you only have to worry about ethernet cords, cards, or wireless cards, and a wireless router. In my area this is much cheaper than multiple phone lines.
read something like The Future of Telephony to get a better understanding of the telepone world and options. That and the cost of various options will help you figure out what you need. Also - fundamentally you need to decide if you want you want calls in and out of your company going through the pstn, voip gateways to the pstn from providers, or both.
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how is your office connected to the PSTN now? assuming you have 5 lines for those 20 people, maybe you have a partial T1? then you would buy a T1 card from Digium or Sangoma and you’d be all set.
let us know how you are connected now and we can give some guidance.
obviously a second person can’t use the same PSTN line if it is occupied. that question doesn’t make much sense 
did you mean same VoIP line? if so, it depends on the provider.
read the book in my signature for an overview.
remember that lines <> phone numbers.
what benefits are you trying to achieve by this switch?
You should check out the IBeX system finite-tech.com/ibex. This is an off the shelf Asterisk based solution.