If you have POTS- say two lines (111.1111 and 222.2222). You advertise 111.1111 as the number for your biz.
Put these two lines in a hunt group
caller dials 111.1111. line1 rings, is answered, etc. second caller dials 111.1111. line2 rings, is answered. third caller dials 111.1111, gets busy signal.
You dial out. call goes out on line1. You dial out again. call goes out on line2, AND CALLER ID SHOWS UP AS 222-2222 unless you can get the phone co to change this (less likely).
With POTS, each line can have one and only one audio channel. Call waiting service is next to USELESS with asterisk because connecting the 2nd caller disconnects the 1st one.
Hunt groups are for incoming. Asterisk does the same thing though using zaptel groups, pick a free line and use it.
VoIP (assuming standard provider type, allowing two channels).
In/Out works the same: first caller calls, goes through. second caller calls, goes through. third caller calls, gets voip provider voicemail.
2nd outgoing call will show correct caller id.
Thus with either setup you can have up to 2 calls going at once. However with VoIP, it costs a lot less, the 2nd outgoing call has correct CID, and the 3rd caller gets voicemail for no extra charge (telco charges $10/mo+ for this service usually).
And on either setup you can have a call queue (callers wait on hold with MOH to get a rep), but each caller on hold uses a line/channel. Keep this in mind.