question

Here's how it works using the example of a two user plan for ABC Company
ABC Company gets a main number and a main fax number.
Then each user gets a separate number (for voice, fax, SMS, etc.)

So that's a total of four numbers:
ABC Company main number 770-123-4567
ABC Company main fax 770-123-8989
User one - John Deer 770-123-8874
User two - John Cougar 770-123-5842
Does that help?

No, Not at all. 

How many calls can I get at the same time?
How many lines does that mean I have because I have 3 DIDs?
How many 'channels' do I have (nomenclature voip.ms uses)? Is that different from lines? 
How many extensions do I have? Or is lines the same as extensions? 
Can I get all 3 DIDs to ring to the first extension then if in use go to the next one or does it have to call wait there first (ie the # of rings I set) and then hunt for the next available one? We're an ecommerce website so we don't want the phone to ring and ring hunting for users. 
What happens when I get 6 calls at the same time, say 2 on 1 DID, 2 on another DID and 2 on the toll-free? 

Sorry but all you told me is I can have 4 numbers, with no explanation of anything that I actually asked. 

I spent quite a bit of time on the phone with a CSR and have finally had it explained to me.

You can get 50 concurrent calls to any user at one time (some call these 50 calls 50 channels)

Your main # can be your toll-free # and you can forward your local DID's to it which results in all calls from all 3 #'s to ring to the same user always. 

You can configure a user's phone to ring for the # of rings you configure it for until it goes to the next user that you want it to go to or to voicemail either for that user or another vm, such as a general company vm or a tech support vm or whatever other vm you want. You can also set those # of rings to 0 so it instantly goes to wherever you configured it for. The latter is good when you have a pool of users that take phone orders because you will want the call to be instantly picked up by the next available salesperson not keep ringing a few times to each one before the call rolls over to the next user if it is not picked up..

Users are not the same as extensions .You can have as many extensions as you want, which will then go to voice mail if they are not attached to a user.

When a user dials out they can choose the phone # they dial out on. You do this by simply clicking the line you want to call out under. There is no need to go into some configuration file first. That is important if you operate in 2 countries like we do (US & Canada) and want it to appear to people in the US like you are calling from your US office but to people in Canada like you are calling from your Canadian office, even though you are always calling from the same place, which could be anywhere really.  

It doesn't matter what 'lines' means now as the above explains all I need to know. 

I think that covers all the questions I asked!